I'm on T-Mobile and recently jumped to the G4 from the Nexus 6. Really loving the G4!
Notable differences for me:
Size - Nexus 6 is big, no two ways about it. Even coming from a Note 3 beforehand the N6 always felt just a tad too wide for me. The G4 feels great in hand yet still gives you a nice big screen.
Sceen quality - G4 all the way. Turning ambient display off on the N6 helps, but I'd still give the edge to the G4 screen
Camera - N6 camera definitely lacks respect! It's camera lived up to my needs. However, the camera on the G4 is just stupid good! The in-depth features of manual mode make me want to take photography classes LOL
Features - I love me some stock Android, dont get me wrong, but (IMO) it lacks features and that leads to rooting and flashing ROMs. Which I used to love doing, but life and times change and I'd rather just simply run my phone nowadays rather than constantly flashing, tweaking, fixing it. G4 has great features that are actually useful! Customizable navigation keys, customizable notification shade quick settings, lockscreen app shortcuts, separate wallpaper on lockscreen, IR blaster, etc. Several features I see on the the G4 remind me of features in custom ROMs.
Bloat - enough can't be said about having a Nexus device free of manufacturer and carrier bloat! T-Mo G4 isn't without bloat, that's for sure but thankfully all of it can be disabled either via the built in method or by using the debloater utility.
Battery - I actually had better battery life with my N6. But a removable battery on the G4 is great. Just got my free battery and charging cradle from LG and now I go from 5% to 100% in about 40 seconds
Sound - front facing speakers really should be the new standard for smartphones! G4 is definitely loud, but the stereo front facing speakers on the Nexus 6 are just awesome!
Connectivity - finding the G4 is maintaining signal in areas my N6 did not. 4G on the G4 is more consistent.
Call quality - both have great call quality. Loud and crisp audio on both.
Root - no root available as of yet on the G4, but honestly I don't miss it! Does that mean that should root become available I won't root? Probably not because I forgot how obnoxious and intrusive ads are! The G4 stock/rooted would make this awesome device just that much better!
That's all I can think of at the moment. Any more questions feel free to ask!
G4'ed it!