I've always used Pro Circuit on all my 125s and 250s since the late 80s'/early 90s'....just my opinion,but I always thought they had the best mid to top performance,and I've always loved the way the PC silencers sound.
I run a complete FMF system(Desert) on my 500,only because I could'nt find a Pro Circuit set at the time...at some point I will buy a PC setup for the 500 to try out.
Sandmaster995 and don46 both make great points....you can't really go wrong with either one,and Pro Circuit does have a ton of championships.
They're both awesome setups...I myself,like the PC best.