Going along with OP - I too am still confused! I am a new(er) member - and began using National as I was tired of the Hertz Taurus. The "pick your own thing" really appealed to me...and in some airports has worked out great. In others...
At BWI yesterday, had the "choice" of 1 car. ONE. (I had rented a full SUV). They HAD done a very nice thing and pulled it right up to the door for me!
Two weeks ago in PHX I had rented a LUX. Had my "choice" of 2.
In each case, it did work out ok - they had a car that was ok for me - but is this usual or am i just having a string of bad luck? (or bad airports -BWI was always the WORST for Hertz!)
I see some posters talking about Executive - is that a different selection of cars?

On another note - the FS SUV is a Durango, and I am (surprisedly) pleased with it. Doing a weekend away with some friends - between luggage and shopping needed the room; but thought it would drive like a truck. It doesn't -very nice drive and ride. Too funny though; no running boards on a very tall vehicle so we had to stop and buy a stepstool for one of my girlfriends to use.... If you are short, this may not be the vehicle for you!