The lower number of turns, the more powerful the motor. Lower turns are faster, draw more amps, and reduce run time. You will need to match what level of motor you run with an appropriate speed controller (one that can at least handle the power you are going to run). If you are going to RACE... then dont just buy... first figure out what class you want to race in, as each class can have a limit on what motor you can run. You will want to buy the motor that fits into the class you want to race.
For example, stock class usually races 17.5 turn. Superstock usually run 13.5, and mod classes are unlimited (run as much motor as you can handle).