its reliability and affordability

1:Map Accuracy
2:POI Database
3:Scre en Size and Resolution

The re are tons of features specific to the different types of GPS units that you may also find important. For road navigation, you probably will want to look for a good 3d view, and text-to-speech support. For the trail, portability is probably one of the most important features. You also want to make sure that your GPS can be easily updated, and will be supported by the manufacturer in the long run.