iPhone 7 for me has some great additions; brighter screen, Taptic Engine, better camera (front as well?), stereo speakers, extra speed (although 6S isn't slow as far as I know).
Best of all; Jet Black. Makes the design look and feel very different.
Changed it. I wasn't even going to mention it, I don't think you're supposed to be aware of this 'feature', just pleasantly surprised when the phone survives a splash. I've heard some mixed experiences on this.
Glad I moved to the 7 from 6s. Verizon's straight across trade deal several months back sealed that deal. Better things IMO:
Water resistance, dual speakers sound nice, the home button, actually like it better. Battery life is better. Seems even a tad smoother if that's possible. I don't use the headphone jack anyway so a non issues for me. The 6s is a solid device so doubt I would have paid out of pocket for the 7.