I have them both I prefer the G4 because of the screen size but miss the performance of the S6. If the S6 screen was larger I would have stuck with it.
I have the g4 and wife has the s6. S6 is a great phone and I can't think of anything bad to say about it. Camera is outstanding and love the fingerprint scanner. For some reason it locked her out of the phone and she couldn't remember the backup password so she doesn't use it anymore! For me the display is a little too small. 5.5 is the new sweet spot. And being left handed I hate side mounted buttons.
Posted with LG G4 via the Android Central App
I think it's going to come down to size first and foremost. I personally can't use a 5.5" phone anymore. I enjoy a more one handed phone.
Posted via the Android Central App