Here's some more color on TUS airport car rental generally, as well as National at TUS. As one might imagine, TUS is a fairly small airport. However, though small, the airport itself is fairly new in appearance. What was really nice about the car rental facility at TUS (which applies to all the car rental agencies there, not just National), is that it is a minor walk (probably 1/4 mile indoors mostly where the very minor incline is eased by a moving walkway and short outdoor covered portion) to the rental facility/garage.
Each car rental agency (the normal 7 or 8) has essentially 2 lanes, with National's being on the far left. As an Emerald/Executive/etc. you don't have to wait at the line at the counter, and walk directly to the garage area, where there is a National stand-alone booth. In line with the others' postings here, when I arrived (on a Wed evening), for my rental (full size) I was given the choice of exactly 2 cars (Malibu or Impala; I chose the Malibu). There were about 10 other cars in the National lanes, but there was no flexibility to get any of the others cars without paying. I was offered an Equinox for $10/day extra (which after pressing him, the agent offered for $9/day), but that was it. When I returned my car (on a Saturday morning), it was likewise sparsely populated. Unlike at most other airports I've returned to, you don't return it to a return line, but rather are expected to park it in a space--any parking spot within their lanes.
In sum, I really likely the setup of the TUS airport and car rental facility (no bus ride to a far away rental facility building where they charge you 50% on top of your rental rate to pay back the costs of building the thing) because it is easily walkable without much effort (and the help of a SmarteCarte). The National selection? Not so much.