I'll always use National. I have Emerald Isle membership, and you never wait in line and never have to even talk to someone - just get off plane, get luggage (not anymore with new disney service), get in car with keys already inside, drive out. Takes only a few minutes to install our own car seat - always bring it, cleaner and comfier, and safer.
It's all about the emerald isle membership that makes it the best, and being at the airport - any other way is a waist of time and hassle that isn't worth the extra savings for us.