For the OP:
1. You'll be best served if you start thinking about Android differently than you do your iPhone. iPhone syncs with your computer - Android syncs with cloud services. Any newer Android phone does NOT need to sync with or ever be plugged into a computer - EVER.
2. Mess with a local music manager if you want - doubletwist, winamp, whatever. If you do that you will be missing out on one of the single greatest features of owning an Android - Google Music. You can store up to 20,000 songs you own on Google's servers. Make playlists, etc. And to "save" then to your device - by album, song, or playlist, you only push the little push-pin icon and voila - it's on your phone. Push the pin again and it's gone.
There is NO device limit, you don't have to authorize computers to play your music - it's anywhere you can login - to the internet. (some country restrictions maybe).
3. There are more iPhones than any other phone. That means there are more cases, cables, docks, stands, chargers, etc. for the iPhone. Accessories can be scarce. The HTC One has a nice car dock, plenty of cases and screen protectors. If you're an accessory hound - iPhone is the only real choice.
4. The HTC One has the best display on a mobile device. It's absolutely stunning and you can see it in broad daylight. There won't be a 1080p iPhone unless they make the screen bigger on the iPhone 6. That will be released fall of 2014.
5. You will not have Facetime or iMessage. If you need those - don't jump.
6. There are very, very few apps that are exclusive to iOS now. But, some are pretty popular and there are a number of games not on the Google Play store.
It's a big step, I'd go with a carrier that has a money back deal/test drive, see if you like the look and feel - because you will love the Features.