Shocks!! the sp has fox shocks. That is the main difference. Better suspension than the indy 500.I think there is more to it than that, i know there is but for the life of me i can not remember. If the price is the same or not much different, iwould go sp, i think the indy 500 are great, however, i really like the xc sp series
that liberty VES motor is FAR more powerful than the indy 500. i would go with the xcsp over the indy any day, even if i had to get a leftover for price. theres a huge difference in them. just about everywhere on the sled is better on the sp.