I'm coming from an older phone (Galaxy S3), so either of these are going to be massive updates for me, personally.
With both phones fully revealed, and both essentially being released to the public at the same time (April 1 vs April 20, I believe), and both being priced essentially the same, I'm really having a hard time deciding between the two. I hate that neither have removable batteries. I recently bought a new battery for my S3 and it breathed life back into the old gal. Over 2 years of daily use kills a battery, and I was getting less than half-a-day use on the old battery. New battery? Back to 2 days usage again. So, it leads me to be a little wary of losing that feature.
I don't switch phones often. I buy one and use the hell out of it, and try to make it at least 2 if not 3 years on it. Longevity is key. And, it just has to work. The slippery back of the HTC means potential for drops. The glass back of the S6 means the potential for breaks. The better camera of the S6 is a feature I will definitely use. But the expandable storage on the M9 is also welcome. The M9's front speakers just make so much sense. But the potentially better screen on the S6 is also a good thing.
I'm super torn right now. I'm not going to use the Edge screen, so I can throw that out. They are both so damn good but also both in the so-close-but-not-quite camp. I honestly don't know which I'm going to purchase. And, it may come down to just holding them each in hand and seeing what feels right, and making a spur-of-the-moment decision and living with it.