It depends on what voltage you’re running it at, first of all. Then it depends on what you’re trying to do with it. If you’re trying to market the car, you’d probably use the Pmax value (and honestly, that’s probably perfectly reasonable because you don’t usually use the entire output of the motor for more than a few seconds at a time). If you’re looking for the maximum it can sustain constantly for 30 minutes (not super relevant in a car unless you’re doing something strange like towing up a very long hill, or maybe certain types of racing), then you’d use the second, smaller number.