Camera
The HTC One A9 sports a 13MP rear camera with optical image stabilisation and support for 1080p video. The megapixel count is similar to the iPhone 6S, which has a 12MP snapper, but it doesn't offer optical image stabilisation. Then again the 6S can shoot videos in up to 2160p, so it wins on that front.
It's too early to fully judge the actual performance of the A9's camera, but it's going to have a tough fight against the iPhone 6S, which can take really great photos.
Round the front the HTC One A9 has an UltraPixel camera, with big pixels to let more light in, so selfies should be more flattering. The iPhone 6S has a more conventional but fairly competent 5MP front-facing snapper.
Operating system
The operating system is perhaps the biggest difference between these phones, as while one runs Android the other runs iOS.
More specifically the HTC One A9 runs Android 6.0 Marshmallow overlaid with HTC Sense, while the iPhone 6S runs iOS 9.
Both of these are brand new versions of their respective operating systems, in fact Android Marshmallow is so new that hardly any other phones have it yet.
They're also both polished, impressive operating systems with apps galore. The choice really comes down to whether you're more invested in Google or Apple's ecosystem.
Features
The main feature of the HTC One A9 is its fingerprint scanner, but the iPhone 6S can match it there with a Touch ID sensor of its own.
The iPhone 6S also has a pressure-sensitive 3D Touch screen, which we've covered in the display section of this comparison. Music fans may want to take a closer look at the HTC One A9 though, as that supports high-resolution audio, so you can get the most out of your tracks.
Battery
The HTC One A9 has a 2150 mAh battery which can supposedly last for up to 16 hours of talk time, 12 hours of HD video playback, or 60 hours of audio playback. That's slightly better than the iPhone 6S, which has stats of 14 hours talk time, 11 hours of HD video, or 50 hours of audio.
In practice the iPhone 6S could struggle to last a day, so hopefully the HTC One A9 will have it beat there, but we'll reserve judgement for our full review.
The HTC One A9 does support Quick Charge 2.0 though, so you can juice it up quickly when it runs dry.
Price and availability
The HTC One A9 costs £429.99 (or US$399 with 32GB and 3GB of RAM if you're in the States) meaning it undercuts the iPhone 6S, which starts at £539/$649/$AU1,079 SIM-free. Then again you don't have to wait to buy the iPhone 6S, as it's already out.
Verdict
While not quite a flagship phone the HTC One A9 looks set to be one of the best Android alternatives to the iPhone 6S. The design is surprisingly similar and the two phones have fingerprint scanners and similar spec cameras.
The HTC One A9 even comes out on top in some ways, with a larger, sharper screen and potentially longer battery life.
But with its speedy processor and innovative 3D Touch tech the iPhone 6S is still likely to come out stronger in a lot of areas.
The HTC One A9 is unlikely to sway many would-be-iPhone-buyers to Android, but for any Android fans who long for a phone a bit more like Apple's offerings this could be just the ticket.
