Apple announced the iPhone 6S and iPhone 6S Plus in September 2015, but despite being an S year, the company claimed it "changed everything about the new iPhones". The iPhone 6S and iPhone 6S Plus succeed the iPhone 6 and iPhone 6 Plus, although the two older devices are available.

Apple fans would have known not to have expected anything too dramatic in terms of design changes for the iPhone 6S and iPhone 6S Plus, but there is more to talk about than we thought there would be when it comes to differences.

We have put the two devices head-to-head to see how the iPhone 6S compares to the iPhone 6 and help you decided whether the iPhone 6S is worth the upgrade.

The design of the Apple iPhone 6S and the iPhone 6 are almost identical. Not much changed in this field. In fact, they look exactly the same with all ports and buttons remaining in the same place for the iPhone 6S as they are on the iPhone 6.

This isn't a bad thing because the iPhone 6 offers a fantastic design with a zero-gap construction and a remarkable execution. There isn't a sharp edge in sight and the same applies for its successor, even if there have been some slight changes to the measurements and weight.

The iPhone 6 measures 138.1 x 67 x 6.9mm and weighs 129g, while the iPhone 6S is ever so slightly bigger at 138.3 x 67.1 x 7.1mm and a little heavier at 143g. Will you notice those fractions of millimetres? Definitely not. You might feel the extra grams of weight if you hold a 6 and 6S in each hand but the iPhone 6S is still a lovely slim and light device like the iPhone 6.

The iPhone 6S is made from a slightly different material than the iPhone 6 however, although still aluminium. Apple opted for the same 7000 grade it used on the Apple Watch. which is said to be stronger and lighter than the aluminium used for the iPhone 6. The extra weight comes in the 3D Touch technology in case you were wondering - more on that in a minute.

Last thing on design is colour options. When the iPhone 6 launched it came in silver, space grey and a new gold colour, but this gold option is no longer available directly through Apple. The iPhone 6S also introduced a new colour when it launched called Rose Gold, but it is also available in the gold launched on the iPhone 6, as well as silver and space grey, offering more options than the older device.

The Apple iPhone 6S and the Apple iPhone 6 both have a 4.7-inch LED back-lit display with IPS technology. Both the iPhone 6S and the iPhone 6 come with a 1334 x 750 pixel resolutions, which deliver a pixel density of 326ppi.

On the whole, this is pretty low for this level of the market, with competitors sitting around the 600ppi mark and one even at 806ppi, but in reality, we have no real complaints about the iPhone 6 or 6S displays. The colours aren't as punchy as competitors but the viewing angles are still great on both and everything looks crisp and sharp on both devices.

Apple did make the iPhone 6S's display a little more interesting than the iPhone 6 though. The latest iPhone features what Apple calls 3D Touch, which you can read all about in our what is 3D touch feature.

In a nutshell, this technology knows how light or hard you are pressing the display in order to bring up a new level of interaction and it allows you to do things like launch the selfie camera by deep pressing the camera app rather than opening it altogether. It isn't just secondary menus on the homepage though - the 3D Touch feature also offers a feature called Peek and Pop. This allows you to peek into things, like an email without opening it fully, while the Pop element opens what you have been peeking at with a longer press.

Is 3D Touch worth the upgrade? It's a great addition and very clever. Third-party apps including Facebook, Twitter, Instagram, Citymapper, MyFitnessPal and plenty of others also have support for the technology which is useful. It requires you to reprogramme the way you use the iPhone so it's a learning curve, but it's a positive one.

The camera has also seen some improvements for the iPhone 6S, moving from an 8-megapixel sensor, found on the iPhone 6, to a 12-megapixel sensor. Apple highlighted all kind of technical terms during the presentation but concluded with the fact that we didn't really need to know any of them and that actually all we needed to know was that the iPhone 6S will take better pictures than the iPhone 6, which turned out to be the case.

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. The iPhone 6S can also shoot 4K video, which the iPhone 6 is unable to do and the quality is good, even if you can't opt for slow-motion capture at this resolution.

The front-facing camera also changed from a 1.2-megapixel snapper on the iPhone 6 to a 5-megapixel snapper on the iPhone 6S, but that isn't the only thing looking to improve your selfies. The Retina display also doubles up as a flash on the iPhone 6S, with the device then analysing the light needed and changing the hue of the white accordingly.

Apple also announced Live Photos with the iPhone 6S, which all Apple devices are able to play but not capture. It's not a new idea, but it is new for Apple users. When you take a picture, the iPhone 6S captures 1.5-seconds before the shot is taken and 1.5-seconds after the shot is taken. When you then go back to look at the image, it will show you the ripple of water or moving cars when you hold down.

Everything else remains the same between the iPhone 6 and iPhone 6S, including the True Tone flash on the rear, auto image stabilisation, auto HDR and features such as burst mode and timer mode.

As you would expect, Apple introduced a new chip under the hood of the iPhone 6S, opting for the new A9 processor with an integrated, always-on M9 motion co-processor over the A8 and separate M8 motion co-processor found in the iPhone 6.

Performance is great on the iPhone 6 but things are that little bit faster and smoother with the iPhone 6S. The latest device also comes with a one generation of Touch ID and it's noticeably quicker to respond. Battery life is also improved on the iPhone 6S.

The iPhone 6S also has an extra storage option of 128GB in addition to the 16GB and 64GB, the latter two of which are also available on the iPhone 6. Apple doesn't offer microSD support on any of its devices so if you need the storage, you've no choice but to splash the cash and pick the iPhone 6S.

Talking of cash, the new iPhone 6S starts at £539, while the iPhone 6 starts at £459 so it is slightly cheaper to go old. 

The Apple iPhone 6S debuted with iOS 9, which was originally previewed at WWDC in June. The new software brings several new features with it including improvements to Siri and search. Apple has also added transit to Maps in the software, which is useful.

The iPhone 6 is also on iOS 9 too though so the software experience on these two models is the same, except for 3D Touch and Live Photos. You can read our iOS 9.3 tips and tricks feature to find out everything the iPhones can do.

The Apple iPhone 6S is the most exciting S model for a long time. It bucks the usual trend and delivers a phone that offers plenty to iPhone users new and old alike. 

The iPhone 6 is still an excellent smartphone but the iPhone 6S offers improved battery life, enhanced cameras and 3D Touch, the latter of which is a great feature when you get used to it. The addition of Live Photos and 4K video on the iPhone 6S might be less integral to all, but they're delivered well.

If you're an iPhone 5S user, the iPhone 6S will move leaps and bounds ahead of what you've come to expect from your phone, but even keen iPhone 6 users will see benefits too.

READ: Apple iPhone 6S review / Apple iPhone 6 review