I have a iPhone 6, had a iPhone 5S. Battery life increase is small but i definitely notice the difference. If that's all you're looking for, i'd say hold off. But i'm happy with all the improvements as a whole.
Don't know about the 6+. But I get 8 hours of usage out of my 6. That's with wifi and 3G using safari, YouTube, a little spotify, and a game or two. I got maybe 5-6 on my 5s with the same usage pattern.