Both the HTC One M9 and the HTC One A9 run Android overlaid with HTC Sense, but the A9 ships with Android Marshmallow, while the M9 currently runs Android Lollipop, which is the previous version of the OS.
An update to Marshmallow is expected for the M9, but until that arrives the HTC One A9 has the edge, with full permission control for apps, improvements to Google Now and more besides.
The HTC One A9 has a more simplified UI as well, taking a lot more cues from Android's stock interface and shedding a lot of the HTC-created apps in favor of the stock Google options to keep the internal storage more agile and sleeker.
The big feature the HTC One A9 has which the HTC One M9 doesn't is a fingerprint scanner, giving you one more option for securing your phone and, perhaps more importantly, making the A9 fully equipped to use Android Pay.
It doesn't have the HTC One M9's dual front-facing speakers though, so don't expect audio quality from the phone to be as good from the mono speaker at the bottom.
The A9 is also offering hi-res audio at 24-bit sampling, so the output of even the tinniest tunes is enhanced. That said, it's not a new trick for today's smartphones, but it's a big step forward for HTC, a brand that prides itself on its audio performance.
The HTC One A9 has a 2150 mAh battery quoted for up to 16 hours of talk time, while the HTC One M9 has a larger 2840 mAh battery which can supposedly last for up to 21.7 hours of talk time.
That doesn't sound too promising for the HTC One A9, especially as in practice the M9 was often running on empty long before bed. At least both phones support Quick Charge 2.0, so you can get them back up and running fast.
The One A9 does have a couple of tricks up its sleeve, mostly coming from the improved efficiency of Doze mode from Android Marshmallow and the lower-power (and newer) Qualcomm Snapdragon 617 chipset, so there's a good chance it could outlast the older model despite having nearly 25% less battery capacity.
Price and availability
The HTC One M9 has been out a while and can now be found from roughly £375 if you shop around. The HTC One A9 on the other hand is launching for £429.99, which puts it in the curious position of being more expensive, despite arguably being lower end.
In the US, the One A9 will confusingly be available for the $399.99, which means you'll be paying less for a phone that has better specs than that available in the UK.
No, we've no idea how HTC thought that was a good plan, either.
The HTC One M9 might be older than the HTC One A9 but it's also higher spec, with a faster processor, a bigger battery and more megapixels.
Then again the HTC One A9 benefits from a fingerprint scanner and comes with Android Marshmallow from day one, so it's a strong alternative, especially if biometric security is important to you.
The M9 is still HTC's flagship, but the sleeker design, improved camera and all-round upgrades of the One A9 could sway a lot of people - although not in the UK, where the price is inexplicably high for a lower-spec model.
- If you wait a little longer you could get the HTC One M10.
Sign up to receive daily breaking news, reviews, opinion, analysis, deals and more from the world of tech.
James is a freelance phones, tablets and wearables writer and sub-editor at TechRadar. He has a love for everything ‘smart’, from watches to lights, and can often be found arguing with AI assistants or drowning in the latest apps. James also contributes to 3G.co.uk, 4G.co.uk and 5G.co.uk and has written for T3, Digital Camera World, Clarity Media and others, with work on the web, in print and on TV.