The answer depends on how technical you want to get using your GPS.
For myself I have 3TOM TOM's now and had a Garmin that I did not like. I have loaded Walmart, Sam's club, flying J/pilot and Camping World POI's as well as National park/public campgrounds POI's into it and I created a low bridge clearance file for my Tom Tom from CSV spreadsheet that was online. I also have gas prices and real time traffic info on my TomTom that automatically will reroute my trip should there be a traffic jam ahead of me.
So how important are these to you?
How big a screen do you need 3 inch 4.3 inch 5 inch or 7 inch a lot depends on where you intend to mount it.
Those are questions you need to answer.