I have run both engines in Z900 Deere's and there's little difference in performance in the real world. If all you cut is super thick, over watered, over fertilized lawns or fields, then you will want the little extra from the 960.
If you found that the SZ held a hill better, I'm betting the pressure in the rear tires on the Z950 wasn't set correctly. They hold a hill as good as anything in the same class/design. I am not sure if the new R series has the cast iron rear bumper that mine does though. It doesn't look like it on their site. It looks to be just a heavy gauge steel bumper. That cast iron bumper makes a world of difference.