I would be biased but, I would race the CR250 for MX and go with the YZ426 for desert. I rode a 91 CR500 around an MX track and was very impressed. I used to race open class a long time ago. That low end torque is just awesome! Still though, that CR500 is pushing 60hp, and sand just eats horsepower. Maybe the CR500 would be better for desert. Put on the heaviest flywheel weight you can find, and go dig some trenches. I don't know, it is a real tough decision.