They are both incredible boats. Both are fast, loose and great on waves. Both are great for all tricks in holes.
I own a Project X 56. My reasons for buying this over the Rock Star are:
I fit better in the Project 56 (I'm 6'1"). More foot room. I like the tiny extra length in the bow and stern for helping to bring a more controllable feel when cartwheeling. I'm over the corky spud boat feeling when cartwheeling -- fast and (nearly) uncontrollable cartwheels. I personally like the WaveSport outfitting better than Jackson. I dislike Jackson backbands with a passion... and this isn't due to user error cuz I know how to adjust them. Two things happen with the Jackson backband: 1) Whenever I go for loops the backband goes under my butt. 2) the rope system tears the crap out of my hips.
With all of that said... I highly recommend trying both. Some people fit better in Jackson, than WS. Some people fit better in Pyranha's than WS. etc, etc. As far as performance both of these boats are about as good as you can get. It will really come down to the little things, like outfitting and comfort, that will help you make your decision. Either way it turns out, neither of these boats will impede your progress of throwing down at your favorite playspot.