1) Design: In-dash or portable unit or smart phone app: In-dash have more functions available and can come already installed in a vehicle. Portable units can be moved from car to car but will have wires that may get in the way.
2) Check the location of the antenna. Try to get the best exposure to the sky for the type of GPS you want.
3) Advanced Features: just navigation or are there other features you might want to use such as real time traffic reports, live fuel pricing, weather reports.
4) Voice commands so that the driver won't be trying to enter information while driving unless you usually have a passenger who can enter the information. Some units lock the keyboard and touch screen while the vehicle is moving.
5) Price and installation
6) Size and viewing performance of the screen: a larger screen may be easier to see and read, but more likely to be affected by sunlight