If you want to use it on your bike, ideally I'd want one that is:
2) withstands vibration (and not just for the first couple of weeks);
3) has a display meant for viewing in direct sunlight;
4) comes with mapping software you can load on your PC, or laptop, to allow you to plan routes along the roads you want and upload them to your GPS (any Garmin Nuvi unit I've seen simply allows you to pick a point to navigate to and gives you the choice of either faster time, or shorter distance);
5) can be operated with a gloved hand; and
6) has an internal battery.
In Garmin's lineup that probably means one of the Zumo's, or one of their marine chartplotters (276C, 376C, 478C).
Car units generally don't meet these criteria. That said, some people use them anyway.