Well, they do not look identical, although physically the dimensions are the same.
The smaller motor (300:1) has a black end cap whereas the larger motor (500:1) has a silverfish end cap:
From what I have gathered the actual motor is the same but the gearing is different so they would have to be replaced in pairs if you went to the high torque motor.
I agree the 300:1 should be used in smaller slides and the 500:1 in larger/heavier slides but apparently that is not the way Thor thinks as my 27.1 with a full wall slide has the 300:1 motors installed.