I believe the only real addition to the AppleWatch with a 6S is LivePhotos support. You'll be able to take Live Photos with your 6S and use them as moving watch faces.
You'd be able to use Apple Pay on both your phone and watch, versus having it only available on your watch now (speaking as an owner of a 5S, and I'm happy I can now use Apple Pay).
I believe the only real addition to the AppleWatch with a 6S is LivePhotos support. You'll be able to take Live Photos with your 6S and use them as moving watch faces.