I had it happen one time to me a couple years ago at LAX, where we booked an Audi S4 cabriolet and they didn't have it when we arrived. They gave us a choice of Jaguar XK8, XJ8, Audi A8, Cadillac XLR instead for the same price, or a standard car for lower price.

Looking a their list of cars, I would agree that the TT is probably the most fun / best choice. Call the station a few hours before your arrival if it's important to you, but be prepared to get surly employees.