[One (M9) General] solsearch:
For me it's various factors.

I don't like the S6 design. and while the s6 edge IS nice it seems a little lacking somehow (plus the front edge doesn't make it as nice to hold as the M9.

The M9 is simply beautiful despite the reiterative design. Too bad the dot view case covers a big part of that up (but it's also a nice and funky case so I appreciate that side of that). On that note I went with silver and gold as it seems to have that beautiful and luxurious feel that I personally don't get from the gunmetal one (though the pure gold would just be ultra gaudy).

Screen: yes, I would have appreciated a QHD screen but I think we need another gen advancement in SoC for it to work as nicely as we'd like (not to mention the wait for 4K to work well). And yeah, I could notice the difference between the s6 Edge and the M9 in sharpness (which I did not expect to honestly). Also, the potential for burn-in on OLED scares me since I buy off contract and don't want to risk that kind of money unless I see an article of how likely it is on mobile devices where part of the UI can be constantly static (the articles I've seen have been for TVs where things constantly change meaning that it doesn't cover use cases like computer monitors or phones). Yes, it's got a cooler tint and it sometimes seems like a tinge more than a cool tone but it's still a great screen and not a bother for the most part.

Ultrapixel: I liked the ultrapixel on the M7 (I didn't experience the purple haze for the most part until like one month before it got stolen and even then it was only sometimes). Now I wish there was OIS (do I ever) on the M9 but having the ultrapixel on the front seems great for late night video calls without needing to turn on the main lights in the room.

SOUND. Gotta go with the sound. I love sound and though I don't quite reach audiophile levels (I have neither the time nor the money) but appreciate it enough to have the M-80 V-moda headphones (would like the M-100s but that'll have to wait until other expenses are covered). The speakers are great. Maybe they're not the loudest but they are not the quietest and they are CLEAR!! This is one thing I also loved of the M7, since the speakers are also the earpiece sound in calls was perfect with the M7 and am hoping it will be with the M9 too (haven't gotten around to calls as I just got it and friends are at work so I don't want to interrupt). My headphones are at a friend's house but I can't wait to try them with the phone.

Experience. I have always had a good experience with HTC's UI. I hated the fact that on the Sensation RAM was lacking, making the UI too heavy but ti still worked better than many others. With the M7 they pared it down so it was functional, good and didn't gobble up resources so it still works well. I don't like Samsung devices generally (personal preference, if you like them that's fine).

I thought about the Flex 2 but it's a touch too big and can't compare on the sound (plus that nagging doubt about OLED).

So those were my reasons.

Oh, and to be sure I also chose the M9 over comparable Chinese alternatives (whether with the Snapdragon 810 or other "high-end" chips) because for most of its history HTC has been good with software updates. Even when it stopped updating the OS it would often still send out security patches for even a year or more after the last operating system update. Meanwhile my Galaxy Note 10.1 (original edition, US version) never got the update despite Version's version getting it (with no explanation).

p.s. I also love the camera experience and Zoe (great fun to be had with those...)