I went the other direction and paddle the Rockstar. I agree they are both similar in how they paddle with a few key differences. I found the Project X really explodes 'up' as opposed to out (which I really liked) but found the stern was catchy (and then flippy) on landings if I wasn't spot on. Sure I'd get over it with more time though. The other factor for me was leg position. I've always been less than keen on the 'straight' leg position of WS boats. The project x is better than previous models (for me anyway) but still not as comfy as the Jackson.
Hope this helps,
Rapid Media TV Guy