Unless one has problems that are serious, upgrading is very risky and inconsistent.
Of the five iPhone 6 and 6 Plus models I bought for the family when they were first released, each update has resulted in slightly different results on each phone. A highly unusual experience.
Normally each year we start with five of the latest model iPhones, I maintain them all, then buy new phones at the next release. I've always had identical very consistent very good results on updates. Never a problem.
But that was then, this is now and my favorite mobile OS is sadly quite inconsistent and buggy. Hopefully Apple will do as the rumors suggest and that's get serious on iOS 9, returning to the fine Apple quality of the past. I know they can do it...
The question is Will They?