Same bike, but the 500w version is electronically limited to 500w output/available to the motor, where the 750w is limited to 750 or so. Same motor, same battery, different internal controller parameters.
From a practical standpoint, the 500 is going to be just as zippy at lower speeds. Where you see the difference will be when you point both bikes up a hill for instance. With 750w available, the 750w bike will have more climbing power.
If that extra 250w is important to you, or it became important at a later date, there are aftermarket kits available that would level the playing field, with increased performance available from EITHER bike! For more info on that potential, look into Bolton performance mods.....