I'm in exact same boat as OP: due for new phone, would like bigger screen, and tempted by 2015 Moto X – specifically, the fact that it'll be ~half the price as the 6S+, and that the screen and camera are reportedly great. We'll have to wait for reviews on those points, of course. (A subpar camera, which cheaper Android phones usually have, would be a nonstarter for me.)
Also, have been playing with my dad's Nexus 5, and I'm impressed by how fast and fluid stock Android is these days. Even the UI doesn't look so bad to me, and I've *never* liked the design of stock Android (and don't care for Material Design on the desktop.)
The biggest thing that gives me pause about going back to the Android "ecosystem" is losing iMessage. My wife and most other friends & family members have iPhones, and it's such a big part of how we communicate now. I'm afraid of missed messages and other issues if I were to rely on SMS (and asking others to use a 3rd-party chat app, just for me, seems impractical).
Same as the above for FaceTime, to a lesser extent. I have a work iPad, which is where most FaceTime chats happen... but nice to have the option on the phone as well. And again, asking others to use Skype or something seems silly.
Chances are, I'll stick with iPhone... but if the Moto X is highly reviewed, a part of me will be wondering hard if I shouldn't go with that and save some serious money.