slow performance wise, I think you would notice quite a difference from 950 to idol 4s... I notice it from my 950xl to X3 but it wasn't enough to really drive me yet. But from 950, it should be more and definitely don't forget the battery...
It's nice to be out and about all day, using the phone often and not worry about battery. I can't speak from personal experience, but reading other comments, it's a win.
I find the complaints about 950 interesting as my wife uses one. And seems quite happy with it. She's not into the app except for say whatsapp and instagram... other than that it's about time wasting games for her. She does complain about battery but acknowledges she can be on the screen a lot playing games. Otherwise it seems to work reliably for what she needs.