I've got an iPhone 5, HTC M8, and an iPhone 6 preordered. I like the iPhone 5 and the HTC M8, but the iPhone is just more useful to me as a phone. I find it much faster and easier to type on, easier to use one-handed, and much more reliable for web browsing (Chrome for iOS, surprisingly, seems to render faster and more accurately than Chrome for Android).
YMMV - I also have an iPad and a laptop that I'm not usually far from. If I was going to just pick one device - well, I'd probably die, but the M8 would be better for a lot of things. The M8 works better for videos, obviously, and its battery life destroys the iPhone 5.
All in all:
HTC One M8
+ 1080p display
+ Great design
+ Great battery life
+ Very fast OS/App loading
+ Very loud speakers
+ Works great with a ton of other Google stuff (Android Wear/Chromecast/Google Services)
= Responsive screen by Android standards
= HTC Android skin is actually okay, not too intrusive
iPhone 6
+ Great design
+ Access to all the newest apps/games
+ Built in FaceTime/iMessage (great for traveling out of the country, assuming you know people with iPhones)
+ Great resale value
+ Integrated health features (M8 has some, but they're minimal)
+ That sweet, sweet AppleCare+
+ Superior web browsing
+ Works great with a ton of Apple stuff (Apple TV/OS X/iPad/Apple services)
= High res, 750p display
? Not sure how the battery life will be
? Could potentially be held back by 1GB of RAM
Ultimately its a pretty close choice, but I think there are going to be a lot of exciting new software possibilities for the iPhone 6 that the M8 won't have. I'm still keeping my M8 though - it makes a great Gameboy DS.