Things change quickly in the smartphone world, with new technologies, competitors, and changing attitudes constantly keeping phone manufacturers on their toes.

The latest flagship Apple device, the iPhone 6S, hopes to remain on top in the ever competitive smartphone world. But does the S model add enough extras to the earlier iPhone 6 to warrant that upgrade, or is it just more of the same?

READ: iPhone 6S Plus review

Apple says that "the only thing that's changed is everything" and having used the iPhone 6s since the start of September we've been keen to see if that's truly the case.

On the surface everything about the iPhone 6S appears the same as the earlier iPhone 6. The form is the familiar finish, complete with curved glass, curved edges, and a polished exterior. But if you read the specs then you'll see the two devices are ever so slightly different: the iPhone 6S measures 138.3 x 67.1 by 7.1mm and weighs 143g compared to the older iPhone 6 which measures 138.1 x 67.0 x 6.9mm and weighs 129g.

So the 6S is heavier and a tiny amount larger, but in the real world you'll be hard pushed to notice the difference in size or weight. If you are worried these tiny changes are going to mean your cases and accessories don't fit anymore, then don't be - we've had no trouble with a range of iPhone 6 accessories we've got in the office.

Apple won't confirm why the iPhone 6S has 14g of extra weight, but the belief is that it's a by-product of the new 3D Touch display (more on that in a bit) and the new stronger aluminium used for the casing (they aren't falling for that one again). The screen delivers the same 750 x 1336 resolution 4.7-inch panel as seen in the iPhone 6.

The other more pronounced change is the option to have the iPhone 6S in a Rose Gold finish, which we think looks very pink. Oh, and it's no longer possible to get the iPhone 6 in a Gold finish, which is reserved for the 6S alone.

So the changes are there, but they don't impact the user experience. The iPhone 6S still looks as good as it did last year, which is a testament to Apple's design approach. Sure, some will moan that the camera protrudes slightly from the rear of the phone, but having used an iPhone 6 for the last 12 months we've not found it to cause too many issues. Besides, Samsung and other manufacturers follow a similar suit with protruding cameras, so while we'd prefer a flush rear the design has almost become the norm across the board.

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One of the biggest changes to how you use the iPhone 6S is the introduction of a new feature called 3D Touch. Working in a similar way to Force Touch on the Apple Watch and MacBook trackpad, the iPhone 6S can now determine how much pressure you place on the screen.

Baked into the core of iOS 9 - the operating system that ships with the device - and available to third-party app developers, the 3D Touch feature opens a host of new options for users to get to information quickly. On the homescreen, for example, pressing down on certain icons now reveals a secondary level of options. Press on the Phone app icon and after a small haptic buzz the screen reveals your top three favourites to call. Do the same to the camera app icon and you get shortcuts - or as Apple calls them Quick Actions - to elements within the camera like Take Selfie or Record Slo-mo. Different apps provide different Quick Actions: Messages dynamically change based on who you are texting, while we love the Photos option that lets you quickly jump to photos you look a year ago.

But it's not just secondary menus on the homepage. The 3D Touch feature goes further in other ways, with Peek and Pop the primary example. This feature lets you peek into things, such as an email to see what it says without fully opening it, while the pop element is a longer press that then opens what you've been peeking into. This works particularly well for quickly glancing into a specific emails from your inbox, while being able to check your calendar to see if you are free - no need to go through the process of opening your calendar app and then double clicking the home button to return to the Mail app. The one we've been using the most, though, is the ability to press on the small Contacts icon within Messages and Mail to quickly access their details.

As 3D Touch can sense pressure levels, it's also come in handy when sketching in Notes and using the new Markup feature in Mail.

We think 3D Touch is a great addition - although it's a feature that will involve you having to reprogramme the way you use the iPhone, at least at first. That's not to say it's a waste of time. 3D Touch is very clever and as it's system-wide it's going to be really interesting to see how this develops into other apps. Developers, including Facebook, have already promised to utilise the feature, although we're yet to see any real-world examples of what that will mean.

The one gesture we've found hardest to perform is the one where pressing on the edge of the screen brings up the app switch menu (usually a double-tap on the home button), which sometimes works, but other times doesn't - probably to do with how we are holding the 6S at the time. Other times we find ourselves cursing ourself when we've forgotten to use the feature altogether. It's all a learning curve, but a positive one.

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Shot with iPhone 6S

While the two cameras on the iPhone 6S look the same as those on the iPhone 6 from the outside, both rear-facing and front-facing cameras have been improved. The rear camera now uses a 12-megapixel sensor, up from 8 megapixels, and comes with the usual Apple tweaks to get the most from the scene in front of you.

Photos are natural in their colours and tones, with the iPhone 6S performing as well as many compact cameras we've seen. In terms of performance it's able to cope well with a multitude of different shots.

We're impressed with the autofocus too - it actually focuses on the subject, rather than trying to make the whole scene sharp - and copes with an array of lighting scenarios including low-light.

Although the default camera app doesn't allow you granular control over things like white balance and ISO levels, there are plenty of third-party apps that allow you to do just that.

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Shot with iPhone 6S

On the front the 6S has a 5-megapixel iSight camera, but most important is a new feature that turns the entire phone screen into a sort of flash for those perfect selfies. Press the shutter button and the phone will analyse the light needed and then change the hue of the white accordingly. We've seen bright clean whites to creamy tones depending on the ambient light situation.

The improvements on the iPhone 6S over the iPhone 6 can be clearly seen, and Apple has continued to move the capabilities of the camera forward in its latest. Shots aren't just bigger in terms of output size, they're better as a result of all those additional tweaks.

Other camera advancements include bigger panoramas that capture even more detail. The main absence is a lack of optical image stabilisation, which only the larger iPhone 6S Plus offers

According to Apple, the problem with still photos is that they don't show what's happening at the time you take the photo. That laugh seconds before or after a magic moment, or the movement in the background of a river or waterfall. Its answer is something it calls Live Photos, which work by taking 1.5 seconds of video before and after the shot to give you a sense of movement to your photos.

Thing is, it's not exactly a brand new feature in the tech world. Look back to Cinemagraph from Nokia, or HTC Zoe, and you'll find similar solutions (ones that aren't talked about much at all these days).

Using the iPhone 6S a spilt second of that captured movement is shown every time you scroll through your images in the Photos app, while a press of the screen using 3D Touch lets you play the whole Live Photo clip.

The iPhone 6S and the 6S Plus are the only two Apple phones that can capture these Live Photos, but it's possible to share them with other iOS 9 and Mac OS X El Capitan users and keep the effect in place. But sending to Android and friends on other platforms won't work - they'll just receive a default still photo instead.

Apple tells us that Live Photos will get even better with an upcoming software update that will intelligently sense when you raise or lower the phone, so automatic capture will take place at the pinnacle moment. We can definitely see the need for that, as a lot of Live Photos we've shot include the footage of us hastily dropping the camera after taking the shot. Another frustration is that audio is automatically recorded, which at times is lovely - a child's giggle for example - but not so great when you've got the same child screaming in the background.

The people we've shown Live Photos to have fallen into two camps: those with kids and those without. Parents automatically get it and love it. Others, at the moment, not so much.

All that extra video adds to their size too, with a Live Photo being around two still photos in terms of storage. While you can offload some of that to Apple's iCloud Drive service (at a yearly subscription) a 16GB iPhone 6S isn't going to stretch as far as it used to (compounding the argument for a 32GB minimum model).

It's not just still photographs that have had an enhancement. New to the iPhone 6S is the ability to shoot 4K video, although you'll have to make sure you turn on the setting buried deep within the Settings app before you start shooting, as oddly it's not possible to do so from within the Camera app. Perhaps that's because 4K is a bit of a space-eater: a minute of 4K footage we shot came in at 375MB compared to 60MB for a 720p HD clip of the same length, so about six times larger. 

Although 4K is very much in its infancy, that is changing fast with the price of 4K UHD TVs coming down and more content becoming available every month (Amazon has just announced the 4K-ready Fire TV with hundreds of 4K TV shows and movies).

The iPhone 6S doesn't feature a 4K screen like the Sony Xperia Z5 Premium, and you can't you stream its content to your television at this resolution via the new Apple TV (that doesn't support 4K either). But that hasn't stopped Apple from being ready when things do start to upgrade around you. While you do wait, you can use the 4K capture to zoom in on the footage you've shot if you want.

Quality is decent. The 3840 x 2160 footage is captured at 30fps, offering smooth playback, but there's no option for slow-motion capture at this higher resolution. If you want to use Slow-mo then that's available at 1080p 120fps and 720p 240fps for smooth playback at up to an eighth of the original speed.

Thanks to Clodge at C S Lewis in Ascot for allowing us to film. 

With the introduction of every new iPhone comes an update to iOS and a bevy of new software features to enjoy. iOS 9, like iOS 8 before it, doesn't do anything to vastly change the experience in the same way the leap into iOS 7 did a couple of years ago, but it does look to improve and enhance what has already been available to iPhone users over the last year. We've reviewed the operating system in full, so if you want the ins and outs then follow the link below.

READ: iOS 9 review: New and innovative or more of the same?

In summary iOS 9 delivers a new font (Apple's in-house one created San Francisco), a stronger focus on search and mobile working, as well as better battery capability from the 6S. There are the obvious additions for iPhone 6S users only too, such as 3D Touch, Live Photos, and the ability to say "Hey Siri" to call up the voice-controlled personal assistant without the iPhone being plugged in.

Other small features that you might overlook have the potential to become the best bits of the operating system. One feature we really like is when an app takes you to another app (say a browser) and there is now a handy back-link where your phone signal is normally displayed. Little things, but it speeds up that journey.

The move to put Hey Siri centre stage is welcomed, and we've already found we are using it to open apps a lot more frequently than when we did before. How long is it before we lose the homescreen altogether?

You would expect all these new features to hammer the battery, but they don't. Apple has not only improved the battery performance with iOS 9, but despite a 5 per cent dip in battery capacity compared to the iPhone 6, we've seen comparable life per charge.

One thing that will help the iPhone 6S last longer than the previous iPhone is the introduction of Low Power Mode. Available to all iPhones running iOS 9 rather than a specific new feature of the 6S, the new mode shuts down all non-essential functions to extend the battery life of the phone. You're asked if you want to turn the Low Power Mode on with 20 per cent remaining and again at the 10 per cent mark, giving you plenty of juice to still be able to order that Uber home at the end of the evening.

In our tests we've been really impressed with longevity. Our iPhone 6S used only 15 per cent of battery in 12 hours with it on, while an iPhone 6S Plus lasted a similar 24 hours for 29 per cent of battery. As ever with a phone, hammer it with the gaming apps and you'll squeeze that juice out faster, but it's all relative.

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Apple has boosted the power of the iPhone 6S, thus making it faster than last year's model and the iPad Air 2 as well. Inside is a new processor, more RAM, and other advances on the speed, efficiency, and technology front. 

The motion (co)processor has now been incorporated into the main processor, dubbed the A9, to save power and increase performance, and for the time we've being using the 6S everything has run smoothly and swiftly.

Whether that's editing 4K video on the iPhone with iMovie, or playing graphically demanding games like Real Racing 3 or Asphalt 8 (it still gets a little hot after a while), we've not noticed any lag or latency issues. To the point: the iPhone 6S works.

Touch ID fans will also be pleased to hear the sensor has been upgraded too: it's a lot quicker to respond, although Apple still hasn't been able to solve the issue of not being able to use it with sweaty hands after running or wet fingertips after getting caught in the rain.

Verdict

An iPhone S update year usually means a couple of new features that most people could take or leave, but the iPhone 6S is the most exciting S model for a long time. It bucks the usual trend, delivering a phone that will offer plenty to iPhone users new and old alike, with 3D Touch a principle new feature that will lead iPhone functionality going forward.

Improved battery life, enhanced cameras, slick operability from iOS 9 all make the iPhone 6S great to use, even if the screen resolution is still behind the current flagship curve. The addition of Live Photos, 4K video and baked-in Hey Siri might be less integral to all, but they're the kind of fun features people want and that Apple delivers well.

For iPhone 5S readers looking to upgrade you'll move leaps and bounds ahead of what you've come to expect from your phone, while even keen iPhone 6 users will see benefits too. The iPhone 6S doesn't disappoint.