I never had the M8, but I thought I would chime in anyway. I just switched over from the S5. Prior to that I had the M7.
One thing I missed when I had the S5 was the dual speakers that HTC had. The Z3 helped fill that void. The speakers on the Z3 is absolutely great. It made it that much better when it came to GPS navigation while driving and Netflix viewing.
The thing I didn't miss was the weight that was associated with the HTC One. S5 was great at that and now having switched to the Z3, I'm glad the weight is still light.
Z3 gave me butter finger syndrome. This thing was slipping out of my hand due to its smooth back. The HTC One had a nice curve that fit in the hands very well. Getting the case for Z3 to be delivered felt like it took forever to arrive. The case made holding the Z3 much better.
The M7 battery life could not stay up with my active daily routine. Even though I switched over the S5 and got a spare battery, I never swapped them out. The battery on the S5 was able to get me through the day plus some. The Z3 easily meets this and have even survived the next day until about noon before requiring a charge.
I found the camera performance is quite good on the Z3. The other night I was able to get a great night time shot which the S5 really didn't too well. When trying to capture photos of my 2 and 4 year olds, the auto focus on the Z3 is quite fast compared to the S5 and the M7. I finally got some usable pictures without the motion blur for our yearly family photo book.
Z3 user interface is a bit lacking when compared to Sense or Touchwiz, which I have now have a better appreciation for either one of them. I guess there are third party apps that can get you a better UI experience.
Battery life, screen size, camera, dual speakers, mag mount charger port, and its IP rating makes this thing a beast while maintaining a sexy sleek look. There are compromises with all phones. I found that Z3 had the least amount of compromises for what I need it for. Honestly, I rotate phones all too often, but until then I'm going to enjoy this gem as much as I can.