The BMW unit is essentially a Garmin Zumo 660 with additional circuitry to work in the BMW cradle, and all of the BMW motorcycle dealerships pre-programmed if something should happen on the road.
The biggest thing with the Garmin Zumo is that it was specifically designed for motorcycle applications (meaning, they expect it to be exposed to the elements; rain, hot sun, fog, etc). They also expect their units to be subjected to more "road shocks" and vibration during their service life. Car units aren't built to those same requirements (though they are likely designed to handle the heat as the interior of a car can become oppressively hot in the summer).
The Zumo series is also "Glove Friendly" meaning you can operate it wearing standard motorcycle gloves; again, not something taken into consideration when designing an automotive unit.
If you don't care about where the closest BMW dealerships are on a long trip, then get the generic Zumo 660 (or any other motorcycle-specific GPS unit).