We used to use National and found it took us longer to get all our stuff over there, wander the EA to find a car, then go to check out. At Hertz there is never a long wait on the shuttle, and I can be driving out of the lot in less than 10 mins. When it is time to return home I do curbside check in and then return the car. I am back at the airport in less than 20 mins.
I have used Hertz at MCO 7 times in the last year and recommend it highly.
It is a myth that you only save time by renting on site.