for motors they are usually measure in Kv, meaning rpms/volt, so a 1000Kv motor, on 1V would give you 1000RPM. In general, the lover the Kv the more torque, higher the Kv the more RPM. The higher Kv set ups are for lighter cars, and lower Kv for heavier. Lower Kv motors usually run much cooler than higher Kv, and have a much wider range of gearing options.

personally, I go the lower Kv, for castle 10th scale is 4600. That way there is less worry of too high temps, and more torque.