The feature I like about the S5 is that it's 'waterproof' with regards to normal exposure (splashes, rain, sweat, etc). BUT, at least with the S3, it seems Samsung has something added to the firmware or bootloader that makes custom roms like Cyanogen somewhere between difficult and impossible to load (Maybe that's been addressed - I haven't followed it since I've had CM on my S3 since like day 2 almost two years ago).
The M8, on the other hand, if it's anything like the M7 that I already have, has a processes through HTC that you can use to unlock the bootloader and load whatever you want.
Although people say the M7's battery life wasn't that great, mine has been stellar since day one. My S3 will struggle to make it to 11pm some days (some days it doesn't even make it to 5pm!), whereas my M7 can go 2-3 days without recharging and always has. It would seem the rated life on the M8 and S5 are real close, with the M8 being 'rated' at 3 hours longer, and the M7 is 'rated' at the 19 hours like the S5. So I think as far as battery life, either phone is a win over the S3.
My main things with the phone are decent battery life, which it *seems* both would have, and the ability to run custom roms, which the HTC seems to win on based off what I know from my M7. The biggest sticking point on the HTC is the non-removable battery, although it hasn't been an issue on my M7. What do you do if it gets stuck?
The other thing with the HTC is at least with the M7, except for blinkfeed (I hated it and was so glad to be able to kill it after one of the updates), the stock rom from HTC is actually pretty darn good IMO - I only rooted my M7 but have been running otherwise stock. The Samsung roms, on the other hand, suck, thus REQUIRING flashing a different rom IMO.
Can anyone who has used either or both (preferably both) of these phones offer any feedback?