I usually keep my iPhone's for two plus years before I upgrade. It seems every time I'm ready to upgrade, it's in the S years so I'm going from a 5S to a 6S. The 6S is so much better than the 5S. I contemplated getting the 6, but only to save $100. I'm enjoying the 2GB of RAM, the longer battery life, and the 3D Touch, which I didn't think I would use often, on my new 6S. I'm really glad I decided to get the 6S over the 6.
It's a way better phone and you made the right choice. Your 6S will steam along easily for the next two years.