In my experience, generally when I rent from an HLE I get the class I reserve. Perhaps on rare occasion I *might* get a slight upgrade (but usually not to SUV).

At an airport location that has Ultimate Choice, you can reserve a mid-size and take whatever they have in the UC area. There could be some fun vehicles (I've seen SUVs, convertibles, etc.) There could also be just mid-sized vehicles.

The exception to this... When I lived in AZ, I rented somewhat regularly (about 10x a year for 4-5 years) from an HLE and got to know everyone who worked there. They found out what I liked to drive and would usually try to get those vehicles for me.