I don't really use 3D Touch so there's that.Why would you prefer the 6+ over the 6s+? The change in thickness/weight is a tiny one, made to accommodate 3D Touch and the added functionality.
But the best way I can explain my preference is to liken it to the same reason that most of us in here prefer PowerPC Macs. There was just something about the 6+ I liked better.
It felt somewhat better in my hand than my 6s+ does. Maybe my dislike for iOS 9 figures in to that to, IDK.