Wow...this is surprising since you were one of the biggest defenders of HTC pre-release when news leaked out that the M9 would be a slightly modified version of the M8. And I was very critical of HTC. Now you are ready to jump ship to the S6, I am moving the other way towards accepting that even though the M9 looks almost identical to the M8, it is still one of the best if not the best android phone out there. I think the evleaks renders spoiled a lot of us for what the final product ended up being.
But I hear you, I have never ever considered buying a Samsung phone in the past but I am taking a look at the S6, espeically the Edge version. Very nice looking phone. The 1440 screen and small battery is a concern though. Plus I just like the buld quality of HTC phones. For now I will hang on to my Moto RMHD from 2012, it is still chugging along fine. But push come to shove if I had to get a replacement now I would probably go with the M9. Of course we will have to wait and see both in person, that could change things.
I think the M9 will be an excellent phone, believed that when the leaks came out, believe that when the phone was revealed, but I think that there's a better product that is going to compete with that, so I'm going to purchase that. I have no doubt it'll be better than the M8 in pretty much every way, but it does have big question marks for me.
What killed the M9 for me was reports of minimal software improvements and reports of sub-par camera performance. Now the camera I understand is non-final software so hopefully that could be improved a lot (and I really hope it is seeing as it's going to be released in Taiwan in a few days) but Sense getting minimal upgrades as well is unacceptable to me. Adding Yelp to Blinkfeed and a themeing engine seems kinda lazy to me. Granted I haven't used the phone yet, but Android Central pointed out it felt more like Sense 6.5 rather than a full upgrade to the software.
But it seems the biggest thing HTC was going on about with the M9 was the dual anodised metal. To me, that's not a big deal. I don't care for it and instead of putting those man hours into hand finishing the M9, I would have liked more effort into getting the camera right or adding more software improvements. The phone just feels complacent to me. Like they had to create a new phone and wanted to put minimal money into making one so they just used the same design (which is still imo the best looking phone design) beefed up the hardware and did small software inclusions. For a HTC flagship it feels like it's had the least work put into it. One X -> M7 -> M8 felt like big, sweeping changes in both hardware and software, but it seems the M9 doesn't.
There was a much bigger improvement from the M7 to M8 than the M8 to M9 which disappointed me. I was expecting more. Now that Samsung have a UI that is actually fast and not laggy (supposedly from hands-on at MWC), a better camera etc, I feel it's the better product for me at the moment. Only thing that's questionable about the S6 for me right now is the battery life. No one at MWC could give opinions on that (for obvious reasons) so I'll have to wait for reviews. I don't care that the S6 screen is 1440p, but do love the quality of AMOLED displays now. The Note 4 screen is spectacular and supposedly the S6 screen is just as good.
If HTC sort out the camera by release here in Australia, then I'd really consider picking it up. Otherwise, I'll move to Samsung. If I had the M7 and were upgrading my phones like a general consumer on 2 year contracts, then the M9 would be an absolutely stellar device (particularly as the M7 may not get Sense 7?), but having an M8 I think I might pass.