The Rad motors, inside, are all 500w Bafangs despite the ones marked '750' that also have the enlarged plug. Internally they are the 500w core. All kinds of drama over this when it was discovered, and gets rediscovered every year or so by a new crop of owners. Rad justifies it by saying the motor is warranted to run at that wattage continuously, which I am sure is true.
Something else to be aware of: Motor wattage ratings aren't particularly meaningful. Wattage is a function of Volts (the battery) multiplied by Amps (the controller). So if you have a 48v battery (whose max voltage is actually 54.6v fully charged) and lets say a slightly below-average 20a controller, then 54v*20a=1080w. The waters get muddied when you start talking about whether the law applies to peak output or the largely-impossible-to-determine 'nominal', but you can bottom line it to this: Your motor is putting out more than 500w on a routine basis regardless of the label on the outside.
Think of motor wattage ratings as an indication of how much of a beating they can take. 500 < 750 so the 750 will live longer if throttled up a long, steep hill, all things being equal (I am ignoring the nylon gears for the sake of keeping things simple). And as a label that keeps you legal. Its not an indicator of the actual power your bike outputs. And as ElevenAD says... there is an upgrade path even for the USA versions that presumably have a little stronger controller themselves already.
