If you watch the Keynote, there are a lot of differences, mostly internal.
Originally Posted by Chris Cook5
Some changes, the screen, battery life, camera(s), water protection, stereo speakers, taptic home button, faster processors (graphic and cpu), more storage options, antennae redesign, etc.. etc.. etc.. Not a big design change, but there is nothing wrong with the current design. Just a year of general improvements and iOS changes. There is NOTHING wrong with the 6S, you're good for many years. I only upgraded from my 6 (not S) to the 7 because my wife wants my 6 since the camera on her 5S is heavily scratched. I would have waited for the 8. But you're safe with the 6S for at least 3-4 years w/o worry (in my opinion), after all the 5s is still supported.