If they are not the race series tire, run the MTR's. THe side walls are super thin on the street GSA's where as the DOT MTR's have a real thick side wall. If they are the race series tire, then go with the GSA's. Both tires are awsome in the race seires form, so either will do.
they will be regular street stuff from tirerack.com.as far as tires go iam not that hard on tires with no flats over the past four years,running 35 inch bajas and the old 37 inch bridgestones.iam a little afraid of running a "street" tire but hate the ride of the others.it gets below freezing here and the bajas run rough for about 10 miles.
I guess it comes down to how much time you will spend in the dirt. I have been to the goodyear racing warehouse in LA and seen the 37 street GSA's. I would never run that tire in the dirt with that thin of a side wall. You can grab the side will with your hand and move it back and forth very easy compared to the dura wall on the MTRs. What size rim are you going to run?