@Hoff
Another vote for staying w/ iOS10.
Battery life is less w/ either 11 or 12 than 10 (see on-line comparisons).
Also IMO the user interface is more Apple controlled in 12.
I have found no apps that need 12, but some need 11. iOS11 brought 64bit apps to iOS hence app developers will be eventually switch from 32bit (iOS 10 and older) to 64bit. This change in file type is what attributes to less battery life and slower performance I would think.
• The iP6 should NOT update from 10 if you use iTunes to backup/restore and have auto update turned off in Advanced Preferences. Look for this ability in your version of iTunes before connecting. If not present you can revert to an older version of iTunes.
• The iP6 will be more desirable to many buyers if you stay at 10 IMO.