As well as running the same software, the Nexus 5X and HTC One A9 also have a number of other similarities, from fingerprint scanners to screen resolution, but there's a lot that's different too. To help you get your head around what's similar and what's not we've put the two phones head to head across all the key categories.
The HTC One A9 has a slim 7.26mm thick metal unibody, giving it high-end appeal. The bezels above and below the screen are on the large side, but with one housing a fingerprint scanner that's understandable.
The Nexus 5X looks less premium, thanks to a fairly plain polycarbonate back, but it does at least have a metal frame and it's fairly thin at 7.9mm thick.
The HTC One A9 arguably looks a bit too much like an iPhone 6S, but it's bound to turn heads, both because of those similarities and because of how stylish it looks in its own right. The Nexus 5X on the other hand is rather more understated.
Both phones have 1080 x 1920 screens, but the 5.2-inch Nexus 5X is a little bigger than the 5.0-inch HTC One A9. That's not a huge difference but could be enough to push you one way or the other.
If you want something compact the HTC One A9 is a better buy, while for more screen to play with you're better off with the Nexus 5X. The One A9 does have the advantage of using the brighter, more vibrant AMOLED technology though.
It's worth noting though that the Nexus 5X uses Gorilla Glass 3, while the HTC One A9 uses the newer and stronger Gorilla Glass 4.
Power and performance
The Nexus 5X has 2GB of RAM and a high-end hexa-core Snapdragon 808 chip under the hood. That processor has four cores clocked at 1.44GHz and two running at 1.82GHz.
The HTC One A9 has an octa-core chip, but it's a slightly slower Snapdragon 617, with four cores going at 1.5GHz and the other four at 1.2GHz. It has either 2GB of 3GB of RAM though, depending on which market you're in. For example, those in the US will get the 32GB, 3GB of RAM One A9, while those in the UK will have to make do with the 16GB, 2GB model.
Ultimately these are both pretty powerful smartphones, but both also fall slightly short of flagship specs.
There are differences though. The HTC One A9 has a microSD card slot with support for cards of up to 2TB, while the Nexus 5X has no slot, giving the One A9 a big bonus when it comes to storage potential.
The Nexus 5X has a 12.3MP camera, while the HTC One A9 has a 13MP one, so not a huge difference there, but the A9 benefits from optical image stabilisation, to keep your shots steady, while the Nexus 5X trumps it for video as it can record at up to 2160p, compared to just 1080p on the A9.
Round the front the Nexus 5X has a 5MP snapper and the HTC One A9 has an UltraPixel one, which is a fancy way of saying the pixels are bigger so it can let in more light.
Both handsets run Android Marshmallow, which is worth highlighting as that's the brand new version of Android and very few phones have it yet.
There are some differences though, as you get the stock version of Android with the Nexus 5X, while the HTC One A9 has HTC Sense on top, which is slick and stylish but not as minimalist as Google's approach.