I think the biggest thing to consider is how the motor is tuned, and how you gear it. A fast stock motor usually wont have torque, so you will have to run a higher gear ratio, where as a torquy stock motor usually will not have the rpm, so you can gear it lower. It seems that most people around here like the monster stock, pick your tuner. Putnam, Fantom, even Team Brood (local to Seattle) Keep in mind that when your running, you want to know if you run out of, say the brushes you use, that you can get more, or your going to be off base till you get more. So for carpet, keep in mind what Rain City has to offer. For pavement, keep in mind what Kevin has or wherever slse you race. I personally haven't been a fan of Aggressive technologies. (which kevin has) Most of the Canadians we race with love the Putnam stuff. I like what I have tried from them. I have been using the monster with red springs, and Putnam Green brushes. it is fast so you have to run a higher gearing to get the punch out of it. I was told that you can use a Putnam blue brush on the negative to keep the motor cooler, and get much more life out of it before a tear down.
Anyway thats my $2.00 worth