6 has Touch ID. Though the 7 has the 2nd gen tech. Regardless, I disagree that it's a gimmick. Maybe you were referring to 3D touch?
Anyway - I went through the same debate as the OP, and ultimately decided to go with a 7 (which should be delivered today). Battery on the 6 was slowly going down the crapper, and the 7 seemed to include enough differences/improvements to make the upgrade worthwhile:
- double the RAM
- water resistance
- battery (as mentioned above - but supposedly the 7 gets a couple more hours than the 6s)
- camera
- 3D touch
- better screen (theoretically, though maybe debatable if you read the threads around here)
- haptic feedback
- processing power (though like OP I'm not sure where that's going to make a difference)
- stereo speakers (which I don't care much about, but apparently speakerphone calls are much better, which would be nice)
- little things in iOS 10 that don't work with the 6 (like raise to wake)
If I had a 6s I don't think I'd bother, but from the 6, the above was enough to convince me to pull the trigger. I also went for a higher capacity model because I was starting to bump up against the 6's storage limits.