In terms of peak horsepower, the CR500 stomps the CRF450. However, the CRF450 does produce considerable power over a larger RPM range than a CR500. The CRF450 power feels more linear, while the hp comes on real fast on the CR500.
In terms of suspension and handling, the CRF450 is more advanced. The difference is more apparent on the track than it is on the trails.
A top-end job for a CR500 costs less than $200. A CRF450 top-end will cost double, triple, or quadruple that. The 4-stroke rebuild is far more extensive.
I don't ride the track much, but that's where I've seen the most CRF450s. They tend to pass me in the air, and I tend to pass them coming out of the corners.
Either bike is fully capable of launching you off a big jump and landing you in the ER.
Here's a link to an asskicker of a shootout. Dirtbike magazine compares a CR500AF (service honda aluminum frame conversion) against a CRF450 sporting a 500 big bore kit.
Here's what one test rider said about the CRF500, "This is the fastest bike I've ever ridden, I can't believe I didn't get the holeshot!"
Yeah, the CR500 is a darn fast bike, wastes a CRF450 sporting a 500 kit.