Menace, the difference is in the mounts that they come with. Email him and ask him what he has if you like but I’ve looked at the two (motorsport and helmet) and I don’t think you’d use either of the special mounts for mounting it on your motorbike helmet.
Motorsports comes with a big suction cup mount for cars.
helmet comes with a strap that will go through the vents.
Here’s a pic of the mounting for each of these:


Also I don’t know about your head but mine vibrates a lot less than my bike. If you have a look at this video you can see a dodgy shot of my gopro mounted under my visor in the screen shot 7s from the end. I like it there because its not distracting, wont get caught and hits less branches which cause the thuds when your viewing.
[video src="http://s721.photobucket.com/albums/ww215/ollie1965/?action=view¤t=Wataganshillclimb09.flv" /]