Xcode's iOS simulator shows 2GB of RAM for iPhone 6s and 6s Plus, 4GB for iPa...
I was amused over at MacR, where someone was griping that the smaller phone also was getting 2 gigs of RAM and it would run better than the Plus. They thought the Plus should get more than 2. LOL You just can't please everyone. I think both will perform fine. Especially with the new A9 chip which will have greater performance and improved power efficiency at roughly the same size as the A8. Add 2 gigs of RAM and you've got two fast powerful and efficient gadgets.