A slightly faster processor, but other than that and what you said, almost nothing. The main thing you should consider is if the better camera and faster processor is worth the $100 price jump, which would not be worth it for most people. Get a 6 plus and use the $100 toward an AppleCare plan. -iPhone 6 user
The 6S plus also has
Bluetooth 4.2 Vers 4.0
Retina Flash and better selfi camera
Second gen Touch ID reader
For me the extra $100 is worth it but it all depends on the user, I have upgraded every year so far and I am happy with the new phone.
I use the 3D Touch all the time it speeds things up and it gives you more options when working in supported apps.
If you are buying unlocked from Apple try the 6 plus S for a week or so if you find you d not need the new features then return to Apple and get the cheaper 6 plus but I think once you try the new one you won't worry about the $100
But what ever phone you choose get Apple care plus
The 6 Plus is cheaper, and the battery lasts longer when on the latest iOS Release. Other than the little extra features like the enhanced touchscreen sensitivity there is very little difference between the two. Because of that I would definitely say just go with the 6 rather than the 6S.