Mostly their firmware. Which parameters you can change (even in blinky class, without boost/turbo, there can be some interesting left), how smooth they deliver the power, strong or weak brakes, etc...

At least for racing, these mostly count more than just the number of amps (once you got enough to run the motor, that is). For speed runs, bashing, or heavy cars (e.g. 4WD SCT), these things might have less priority than amps.