Re: upgrade to Galaxy s6
I had the 2013 and 2014 Moto X. I switched to the Note 4 about two weeks primarily for the battery. Two weeks in on the Note 4 I can honestly say TouchWiz is not as obtrusive as I expected, and it will be less so on the S6. In fairness I am running the Google Launcher and I probably only use a fraction of the features Touchwiz actually offers. I use my phone a lot on speaker for conference calls. One TW feature I like a lot is that it brings to the notification area options to end the call and go on / off speaker (might have the option for mute too but I don't use that a lot). Small thing, but on the X you would have to pull down the notifications, click "return to call" and then do it in the main phone app. Other item about TW I like is you can customize the quick toggles in the notification area for those widgets. You can pick five from a pool of 25 or quick toggle widgets. Always frustrated me on stock Android that you don't get that option. So those are my thoughts on TW....
Separately, the camera will be way better, as will the screen. One can debate the merits / need for qHD on a phone, but the screen on the Note 4 / GS6 is night and day better than the Moto X. It's actually one of the first things most people comment on when they look at my phone.
I miss Active Display on the X. I'm using an app called Dynamic Notifications which is similar, but Active Display is better. I also use an app called Agent to replicate Moto Assist, and in my opinion, it's better. A few more options in it to tweak just how you want relative to Assist.