The geometry is different. The SS has a very pronounced lip that is supposed to help stretch the skin a bit, but in practice kind of forces you to use a slightly steeper angle of approach. The SR is the newest model and tones down the lip quite a bit so it feels a bit more natural to use. The SR also has more weight to it. I generally prefer the SR after getting it but revisited the SS again lately which was fun to use as well.

Here is a great page comparing the various offerings.