Be aware that the most significant (to riders such as we are) difference between the Garmin automotive GPS units and the Garmin motorcycle specific automotive units is that with the motorcycle units, you can create a "customized" route on your PC or Mac using Basecamp and then download that route to the GPS. In other words, you can tweak the path that the route takes on the computer until you have exactly the route layout you want - then transfer that route to the navigator using a USB cable.
The majority - if not all - of the automotive units do not offer the ability to have a route that has been created on a PC or Mac downloaded to them.
To me, that is the key difference between the automotive & motorcycle navigators. Things like waterproofing, twisty roads, fuel monitoring, etc. on the motorcycle units are 'features', but they are not necessarily benefits.
So, to sum up - before you buy an automotive unit, be sure you can download custom routes to it (and later activate them) if that is important to you.
Michael