All of the posts here are pretty valid. If you can only afford the 16GB Plus, then just get the 64GB regular.
But going back to the question of people moving from 6 to 6S Plus, I made that move. Honestly if these phones weren't so slippery, it wouldn't be an issue at all! I have pretty small hands, and it took some re-training to figure out how to use my regular iPhone 6 one-handed. Now with how my hands are used to holding the phone, it really isn't that different going to the 6S Plus. That being said, get the Apple leather / silicone case. The other cases I've tried are typically too bulky (I know there are some ultra thin ones out there too). Forget the protection. I just need the case to make it grippy! The Apple leather case looks really nice on the phone, and it helps to make sure I don't drop it. In terms of in my pocket, I do notice that the 6S Plus is slightly larger than the 6, but so far it hasn't bothered me too much...