RES stands for reverse elliptical sidecut. Basically, starting at the tip, the ski is fully reverse sidecut, much like a spatula (or a waterski). However, instead of being reverse sidecut all the way through the ski, the widest point is halfway down the tip, it then gets skinnier towards the waist. After a small section of transition, the ski from underfoot down has a more traditional sidecut.
On hardpack the widest point of the ski in the tip creates a contact point with the snow, and in combination with the sidecut in the tail, it will carve similar to a traditional sidecut ski. However in soft snow, it skis much like a reverse sidecut ski, with all the benefits of being able to pivot and surf powder.
I think that the 12/13 ski is much better than the 11/12. If you're looking to tour a lot on it, get the tour layup, but if you're trying to just have a good inbounds ski, it's not necessary to get the tour layup