Having all three (125,250, and 500) I'd have to say that the 500 is the most reliable. It is also the most comfortable, easiest to ride, fastest bike on the planet and just about everything else you can imagine. The diffrence, in my oppinion, with the Cr5 and the Kx5 is that the KX seems to have a bit more pull up top as the CR tops way too fast and has no power up top, where the KX is just pulling away. I'm assuming due to powervalves.
The problem I've and everybody else with a tuned KX is the jetting. It is almost impossible to Jet a KX500. When I first purchased the KX I sent it directly to Eric Gorr for some work, which worked out beautifully, and race tech fixed up the forks and suspension and I can say it is the best bike overall I've ever ridden and is my #1 pick when I hit the trails. :yeehaw: