I've used both Dollar and National out of MCO...
National was very pleasant... Recieved a nice Grand Am to drive to VB... We booked it through them though because we were finishing off our trip at BWV, and they had that Car Return near the MK which made it really convient...
Using Dollar was also very good.. We rented a Grand Caravan from them in November... Went online got a promotional code... Called Dollar's 1-800 # and booked... Simple.. We also tried through MS, and the Disney Club, but they couldn't match the rate we got... (I'm not sure if Dollar has the checkin' though on WDW property.)
I'd recommend either of them though...