Well the Camaro SS is faster than the Mustang GT, so if you want speed, I'd go for that. Other than that just go for whichever one you prefer. The styling is a little different for each. I do know the Camaro is harder to look out of. Both would be nice, but if I had the funds I'd probably do the SS just because of the speed.. plus Camaros are known for excellent handling around the track these days.
Magnetic Ride suspension.. Haven't seen it or heard much of it, but it has to be good if Ford bought the rights to use it on its GT350. If you plan on upgrading wheels or suspension on a mustang I recommend just getting the premium package and skipping on the PP.