HTC has faced tough competition in 2015, with the flagship HTC One M9 - launched in March - not getting quite the reception that HTC had hoped for, despite offering plenty of refinement.

A new entry into HTC's line-up is the HTC One A9. It's a new class of device, a step down from the flagship in some cases, but bettering it in others.

We've lived with both, we've reviewed both, and here we're pitching them head-to-head. So what, exactly, is the difference and which is the best smartphone for you?

The HTC One A9 and M9 both come with a full metal body. The One A9 measures 145.75 x 70.8 x 7.26mm, so it's fractionally larger than the One M9's footprint, but slimmer. The One M9 measures 144.6 x 69.7 x 9.61mm. The A9 weighs 143g, lighter than the 157g One M9.

That slimness makes a difference. The One A9 has a flat back, whereas the One M9 is curved. The A9 is more minimalist, leading to smoother results. The M9 might involve more precision in manufacture, but the A9 feels nicer in the hand.

The build quality of both the One A9 and One M9 is fantastic, but the One A9 feels more considered, even if it does look a lot like the iPhone 6. The HTC One A9 comes in Opal Silver, Topaz Gold, Deep Garnet and Carbon Grey. The One M9 comes in Gunmetal Grey, Gold or dual-tone Gold on Silver. Of the two devices, we prefer the look and feel of the A9.

The big difference between these two One handsets is in the internals. The HTC One A9 is powered by a Qualcomm Snapdragon 617 chipset, with either 2 or 3GB of RAM (depending on territory). The HTC One M9 has a SD810 chipset, with 3GB of RAM.

The SD810 is a more powerful chipset with a more powerful GPU, so the One M9 is better suited to more graphically intensive tasks where the A9 is a little less adept. Both are snappy thanks to great software optimisation however and the SD617 is one of Qualcomm's very latest chips.

That slimmer build in the A9 comes at the cost of battery capacity, with a 2150mAh battery, compared to 2840mAh battery in the M9. However, this doesn't result in miserable battery performance, as the One A9's optimisation both on hardware and software makes it more power efficient than the M9. Both will just about see you through the day, but the A9, for the capacity of battery it contains, is surprisingly good.

Both handsets offer a microSD card slot, but there are two versions of the A9: the 2GB model has 16GB storage, the 3GB model has 32GB storage. The HTC One M9 has 32GB as standard. The HTC One A9 launches on Android Marshmallow, meaning you can use Flex Storage to expand the internal capacity by integrating the microSD card storage. This may or may not be supported on the M9 following an update to Marshmallow.

Both the A9 and the M9 have a 5-inch display and both offer a resolution of 1920 x 1080 pixels, for 440ppi. Technically, both these displays should be equally capable when it comes to resolving detail.

However, the HTC One A9 uses an AMOLED panel, whereas the One M9 is an LCD panel. We found the One M9's display to be fairly weak, not living up to the expectations of previous devices like the M8. The A9's display on the other hand offers the same resolution, but lovely rich colours with real impact that betters the One M9.

For everyday tasks, that means that the HTC One A9 in many cases looks better, with deeper blacks and pop to colours. Some might find it a little less realistic, however.

Both the One A9 and the One M9 have the 4-megapixel UltraPixel camera on the front for selfies and the performance is pretty much the same. But they are different on the rear, however.

The HTC One A9 has a 13-megapixel rear camera with an F/2.0 aperture, offering optical image stabilisation. The HTC One M9 has a 20-megapixel F/2.2 camera. The result is that the HTC One A9 will give you better low light performance, reducing some of that handshake. Neither are the best cameras on the market, but you don't lose out picking the lower resolution A9.

There is one big difference in video capture however. The HTC One A9 offers Full HD video, but the One M9 will give you UHD 4K as well, so if you're a fan of video, you might want to opt for the One M9.

There's a new camera app for the One A9, but this will be updated on the One M9 to offer the same interface.

The HTC One A9 launched on Android 6.0 Marshmallow, with a special version of Sense 7_g. The HTC One M9 (currently) sits on Android 5.1.1 Lollipop with Sense 7. The One M9 will be updated to Marshmallow (reportedly) in December.

However, there's a pretty big difference in the HTC Sense skin over the top. On the One A9, this is closer to stock Android, removing some of the bloat of Sense - there's no HTC Music app, no bundling of Facebook, no additional internet browser, for example.

The aim with the HTC One A9 has been to offer a software experience that uses much more of Google's great offering in Android 6.0, and it's a triumph. The software experience on the HTC One A9 is much better than the M9, benefitting from embracing the best of Android. It's also well optimised, so the performance is really fast.

We're yet to see what will change on the One M9 when Marshmallow arrives, but we suspect much of the Sense 7 experience will remain as it was before.

One of the great elements of the One A9 is the fingerprint scanner. One of the biggest additions on handsets in 2015, it's gives really quick unlocking, as well as supporting Android Pay. It's really fast on the One A9 and we found it to be really reliable.

The One M9 doesn't have a fingerprint scanner.

The One M9, however, has an IR blaster, meaning you can control your TV, for example. This feature isn't on the HTC One A9.

The HTC One A9 ditched BoomSound speakers for a single speaker on the bottom of the handset, as opposed to the M9, with its front-firing stereo speakers.

Instead, the One A9 offers superior headphone performance, dubbed BoomSound Headphones. This uses the power of Dolby Audio, as well as a superior DAC to offer better sound quality for those on the move using headphones, if you can tell the difference.

The HTC One M9 is easily the better handset for listening to music through the speakers.

The HTC One A9 is £429.99 SIM free in the UK.

The HTC One M9 was £579.99, but can now be bought for as little as £409 direct from HTC in gunmetal, but the silver/gold dual tone handset is still £579.

The HTC One A9 differs from the M9 in a number of areas. The M9 sits in the top spot as the bone fide flagship, but the One A9 offers matching build quality and superior design in our opinion.

The One M9 is the more powerful device and has the larger battery, but is known for getting hot. The One A9 is powerful enough, although gamers might prefer the M9's power. That said, there's plenty of appeal to the AMOLED display of the A9.

With the cameras it's time for change and here the A9 trumps the M9 unless you're looking for 4K video capture.

Combine that with the latest from Android and a version of Sense that adds most of HTC's enhancements without the bloat, and this quality mid-ranger has more than a fighting chance against the flagship handset. That might change a little with the update of the M9 to Marshmallow, and we'll soon know what HTC has in stock in that department.

It's worth checking the prices, however, as the HTC One A9 is on the expensive side and the M9 is falling in price as the older handset. The A9 is lighter and fresher. It's a pricey mid-range spec device, but there's a lot to love about it.