If you like a large screen and are Okay with touch screens, I'd spring for the Zumo which has a number of design features tailored to motorcycle use. 2nd choice a waterproof Nuvi. I'm not familar with the rest of the Garmin line, but it seems like the Zumos and Nuvis are getting the lion's share of development resources these days. (The number of models is mind-boggling.... and confusing.)
If a small screen doesn't bother you, you're comfortable with buttons and don't mind a "techie" GPS, the 60 and 62 are hard to beat. I travel with groups that have four or five GPS's among us and I consistently can do a better job of navigation and have more information available to me with my lowly 60CSx. And I have seven bikes and don't want to spend a fortune on dongles and mounts, so long runtimes (using common AA batteries), weather resistance, portability (I can throw it in the tank bag at stops), and PC connectivity trump features like touchscreens.
I'm climbing my soapbox now, but I also find the Zumos and Nuvis so "dumbed down" I feel like I'm being treated like an infant. If feels to me like when Garmin designed these units, they designed around the person who drives major highways and above all else, wants to be able to point to Abilene, say "take me there", and be done with their interaction with the GPS. Those of us who like to plan routes, don't trust GPS routing algorithms, and like to experiment a little get short shrift in my opinion.
- Mark