The 9003 and 9012 motors are almost identical in performance, but they have a notable design difference. The 9012 motor has a current limiter built in (I think it is a thermistor, but I’m no electrical guy) that limits the current under high loading conditions. The 9003 motor does not have this feature.
For performance numbers on both motors check out the “motors” forum and look for the thread about 2006 KOP motor stats as both types were provided in the kit last year.
We accidentally got the 9003’s in our kit this year and built and tuned our system using them. At our first event we swapped in the 9012 motors and everything worked exactly the same. Unless you are interested in/worried about the current limiting feature, I would consider the motors the same.
The current limit is a thermal breaker. It will open when it achieves a certain temperature. This will occur by the passing of a high current or from external/internal heating from the motor and surroundings. Once it has reached trip temperature and cooled, it is more likely to trip again. Many teams experienced this phenomena during arm build and test this year.