I sold a 5S and have a One M8 now and can tell you wont i don't miss for what its worth:
No more piss ant 4" screen.
No more pushing home button to check missed calls/messages all day long. So nice not to have a home button full stop.
No more iMessage for iPhone only. Can message anyone with airdroid off my macbook and iPad.
No hit and miss touch id, double tap to unlock is sweet.
One M8 feels way nicer in the hand, 5S never felt nice naked to me. My wifes IP5 feels buggy after this phone, it is so fast. LTE is super fast.
Call quality and reception is great, speaker is unbeatable.
The on and off button and the volume buttons are smoother than 5S.
No problems with bluetooth in car and office.
Video is great haven't really tested the weak point (camera yet)
This is my first top end model HTC and or Android phone so its all new to me, everything else i own and my family have is Apple. So i am not a Apple hater, love my MBPr and iPad (would never change them). My phone is a business phone which i mute a lot after hours except for family, and is so good to have a notification light again.
Obviously the 5S is 6 months old so the 6 will be a new ball game. But Apple really only release a new phone every 2 years. The S models are just upgraded with gimmicks.
I would say if you are after a social phone get the 5S, if you want a workhorse business phone get the HTC one M8.