Having BOTH at the moment, I vote for the MTR's. I have the 35x12.50R15 on my Blazer for hauling ass to denver, doing burnouts and trying to lift my front tires off the ground. My MTRs are trial 37s and are trial tires at the moment but as soon as the ProCOmps are bald, the MTRs are gonna see the street use. IN all honesty, the MTRs handle better (even though they are bigger), look better and wear better. At low speeds (15mph and less, they are quieter than the Xterrains, but at higher speed I can't tell the difference.
\
I jumped on the Procomp wagon like 2 years ago and did the buy three get one free deal from 4wpw. They are a decent tire but pale in comparison to the MTR.
Chris