If you ride a lot of trail the 6spd 500 is probably a better choice. However, around here where we have very beat up short trails (maybe a couple miles before you never see one again for the rest of the day), the 450 is the right animal for the job. The sleds will lose me in the straights, but the second a few curves get thrown in I rail right past them. I'm almost always first to the end of the trail when I'm with sleds.
The steeper, the more gnarly, the more technical, and the tighter things get the better the race 450s of all manufacturers run. If you have wide open spaces with lots of super deep powder and long hauls to cover the 500 will probably be your weapon of choice.
In the end it all depends on personal preference. Both bikes will get your rocks off.