If there is a good price on National for a mid-size you can use their "Emerald Aisle" program. It is a good deal, you don't have to wait in line at all and you can pick any car in the designated aisle. In WDW last summer we got a really nice, high end SUV for the price of a mid-size (great because we had it for over a week). To use the program you have to belong to the Emerald Club which usually costs some $. It appears to be free on their website just now.
We have rented at both National and Alamo (I believe they might be owned by the same company) and been happy with both.