The real question is what do you want the gps to do for you? Do you want a digital speedo, turn by turn, or stuff we probably wouldn't understand? The bike specific ones do things like record your heart rate and the torque you're putting into the pedals if you want to cough up some $$. It'll do stuff that's useful to a motorcyclist too like some of them have good maps.
I know people who use a Garmin cycling GPS for skiing, they just don't make use of some of the functions like cadence. But every one of them does it because they're also cyclists and already had the bike GPS, so it didn't cost anything to put it to a second use.