Switched from a 6+ to a 6s.
6+ 144 minutes screen on time (SoT) 45% left
Extrapolated out to a 0% run down is 262 minutes of SoT
6S 131 minutes SoT with 16% left
Extrapolated out to a 0% rundown is 156 minutes of SoT
Granted this is the first battery cycle I'm doing since getting and charging to full, and I was downloading a couple more apps and stuff on it to finish setting it up. I would reasonably expect 3 hours of SoT with my normal usage after all is said and done. My 6+ would probably get me around the 5 hour mark. Both of those estimates are going completely to 0% in a normal day for me (about 16 hours unplugged).
I use screen on time because it is a better metric, in my eyes, of actual usage of a phone since having the screen on is what I care most about.