Gosh, if there is no cost savings then skip the rental. I imagine there is no time savings with a rental--compare picking up a car to simply catching a taxi. Then there is also stress savings or having someone else do the driving. Skip the car!^ If there was a cost savings or wanted to explore (even to drive to Animal Kingdom), then maybe.

On recent trips to DizzyWorld we have used a private car company (like Mears). They have always met us at the airport. We had to wait a few minutes to return to the airport one time; the other times they were already there at the requested time. We took Magical Express once and I would not do it again--unless going directly from the airport to the cruise. The Magical Express experience gets you ready for the Disney way (line waiting).