Like others said, both bikes have the same HP. Matter of fact, the '07 S4R had the same power as the S4RS for '07 and '08. Adding the S to S4R gave you Ohlins and I think better brakes. Of course if you find a good S4R for a cheaper price you could buy much nicer Ohlins parts than came on the S4RS. Any of these bikes will be sure to put a grin on your face!
