First up Ollie, I would get one with a built in battery, the vibration on bikes causes shut down problems in units which take disposable batteries.
Then decide if you want colour or B&W maps, I recomend colour for more detail.
Also do you want the ability to load non OEM maps?
I have a Garmin GPS60, a basic model with B&W screen and no maps, just trail and POI map like screen. I really only use it on the boat now, but it came with the Garmin mapping software which is very handy for recording the tracklogs and viewing them on Google Earth etc.
The 60 series dont have a strong reliable bkie mount bracket unless you go for an expensive aftermarket one.
My other unit is the Garmin Quest (not Quest 2), it has built in battery, water proof, colour maps, talks to you and computer, loads non Garmin maps if you want can plan off road routes using your own waypoints (point to point) and has handled being on my DRZ quite well with the motorcycle mount.
Note the Quest 2 does not have the ability to load non Garmin maps.
Only problem with the Quest is the small screen size.
Note 2 the Quest has been replaced by the Nuvi 500.
Get yourself out to Mona Vale and talk to Brian at
GPS OZ he is very helpfull.
Also, I have read on ADVrider that the colour magellan units have slightly better off road detail in their maps.