S model is the "base model" that I will use to compare the other models too.
SS model is a long model. It is somewhere around 4" longer if I'm mistaken and holds another 1/2 gallon fuel. Ride height is the same as the S model
SCG model is the low model. It rides lower and closer to the ground. Not only is the suspension a little bit lower but the stock seat is cut down more. Many find this the more visually appealing bike. Fuel capacity is the same as the S model.
As far as my knowledge goes, those are the differences. Numbers aren't exact, but should be close!