In bought a Garmin model in the very early days and there are some aspects which would be worth checking:
How tough is the screen cover. I use mine for plants and it has got scratched making the display hard to see sometimes.
What is the display like in sunlight.
How quickly does the GPS tune in to satellites mine takes ages when I've moved more than 30 miles or so.
You can get GPS on some smart phones too these days I think.