Another way of looking at this: if the iPhone 6S gains:
Sapphire glass
Faster LTE
Wireless charging
Etc...
Would this be enough to cause you to upgrade from a 6 to a 6S?
For me, I really don't know what Apple could possibly put in the 6S to want me to upgrade from my (coming) 6. And I upgrade every year.
In everyday use, you're not going to be able to tell the difference between a sapphire front and glass. Wireless charging is more of a cumbersome gimmick (you're still going to have to put it on a bulky mat). And current LTE is blazing where I live anyways.