My wife and I went to SW last year, and our rental car had GPS in it (coincidentally). We loved having it. When we wanted to find a place to eat, the unit would pull up a list of restaurants and could tell us how far away they were and in which direction.
The thing to remember when renting a vehicle with GPS is that it is a per day charge. If you are only renting the vehicle for one day, then it's only an extra $10. If you are renting the vehicle for a week, you would be looking at an extra $70.
If you have a multi-day rental, but only want the unit for a day you could rent vehicle 1 for part of the trip then return it and rent vehicle 2 for the second part of the trip. But it may not be worth the hassle to try to beat the system like this.
When in doubt, try it out.