My opinion - I would say that as long as you have an iPhone 5 or later, you should be able to get by well enough for right now, as iOS 9 seems to run alright.
But I would also say that if you have an iPhone 5S or earlier, the jump to the 6S should be significant enough to warrant upgrading. I know the jumps from the original to the 3GS, from the 3GS to 4S, and from 4S to 5S were all quite significant. I see no reason why going from a 5 or 5S to a 6S would not continue this trend.
As great as the 5S has been over the last 2 years, even hanging tough with the 6 as far as performance goes, I really am eager for not only a bigger screen and new features. Plus the features exclusive to the 6 that I missed out on last year such as Apple Pay and VoLTE.
But that's me, and despite all the 6S related leaks, I wonder what surprises Apple still may have up its sleeve for next week?