I'd use Dollar again, mostly because National's prices have gone through the roof, and it's almost impossible to get reasonable rentals from them. Now, keep in mind that my husband insists on renting a convertible when we're in Florida, so that may have soemthing to do with it. We have to book a convertible, esp since picking on the Emerald Aisle seem to be very slim when we've been there. He'd be so unhappy if we didn't get a convertible that I wouldn't want to be on vacation with him, so I always book a convertible level car.
However, I used to be able to get pretty reasonable prices with National, even when renting a convertible. Last few times I've needed to rent and have checked National, the weekly rental prices were in the $400-$500 range (more than double what I've paid with National before).
So I used Dollar because they had better prices. The line thing did bother me, but we still got through the Fastlane line faster than if we'd been standing in the normal line, so if nothing else, we got a much better deal with Dollar, and got through the line faster than we may normally have.