The only comparison I can give is the the standard Rival vs the S.
The S is significantly better in forward bite and turn in. My car has big power so launching it at the auto-x is always difficult, the first run on the S I nearly stalled it at the starting line because the grip was so much better. There were some durometer tests done in Nashville comparing the latest Falken and the S. Falken=55 and the S=51. A friend of mine was able to cut 1.9 60 ft times at the NJ pro solo on a unprepped surface in a stock C6 Z06 on the S.