I don't know much about Thrifty, but do know there's nothing wrong with Dollar. Join their express program, show up at the Express locations, look at the monitor to see where your car is, and if you don't want what they give you, choose another one. It doesn't get much easier.

I've never understood the locations where Hertz is so much more money than some of the other rental companies, but there have been more than a few times I've been walking through when I rent with somebody else because it's a third the cost, and see the "No Vehicles Available" sign on the Hertz counter, so it works for them.