Personally, I chose the 6s+ over the 6s for a couple of reasons
1. Battery life is supposedly better with the plus
2. Camera and OIS!
3. I do a lot of homework and research for my classes, so having the bigger screen is a plus (pun intended) for me. There are those times that I don't feel like taking out my laptop or I just need to look something up quickly. My Droid Turbo has a 5.2in screen, so when I played with the 6/6+ in the store the other day, the 4.7in was a huge downgrade for me. It was so weird to hold a "tiny" phone. The 6+ felt more natural, as 5.2 is only .3 away from 5.5, rather than the .5 it is going down to the 6/6s. I also think a bigger phone is easier to hold when you're talking on the phone. Your hand isn't as scrunched up and its way more comfortable.
Again, it's up to you and what will best suit your needs. If you watch a lot of videos, do homework/research/reading, take lots of pictures, etc., then I'd head for the 6s+