As far as the GPS units that are made for cars... Garmin is better than Magellan because the Garmin units will let you avoid unpaved roads. You don't want to end up like the Chetien's in Nevada (who were using a Magellan).

Beyond that, I think you have to dive into the details and figure out which feature is the game changer for you personally. For instance, the new Delorme PN60 is awesome, but doesn't have an external antenna jack. That alone might be the game changer for you. Or maybe you need long battery life or you need to be able to save a large number of waypoints. The devil is in the details and there are so many models now you just have to figure out which features are important to you.