A killer PSP exclusive racing game? That doesn't exist. Midnight Club 3 is the best PSP racing game. The load times from off the UMD are awful. Worst on the system. Like 70 seconds to load a race. But from off the memory card, the load times are manageable and the game becomes infinitely more playable. Even with bad load times, i must have put around 30 hrs into this game. Assuming you haven't played the hell out of the console version, MC3 is still the best.
I like Need For Speed Carbon, although all the tracks look pretty much the same and there are no distinguishing landmarks are monuments. You won't be able to really tell any one race from another, but I still enjoy the actually racing, and I get a kick out of the crew system, despite the fact that it's somewhat buggy. I wouldn't say the crew system is broken, like some people have suggested, it just doesn't work the way it way designed to all the time. I still think its fun.