In 11 they have the same motor ,prior to that the "S" had a little higher compression,better cam,different clutching, hotter ECU and a better header. This accounted for about 5 HP difference over a non s , however when you count in the extra weight of the tires(bighorns) then the performance was about equal.
The main advantage to the S is the higher ground clearance ,better ride and ability to run large tires with out a lift.When you lift and put offset/spacers on the reg rzr it is quite hard on axles/tierods/hubs etc. Where as on a S you don`t have to offset your wheels to run big tires ,so your axles stay at a much safer angle ,thus being easier on the whole machine.....
The down side to the S is being slightly wider ,about 61" with big tires vrs the reg rzr witch sits around 54 to 56 depending on tire/rim set up.
I have owned 2 reg and one S and much prefer the s because of the ride and stability, but it depends where you want to ride, i guess.....