We've done the car rental from Budget, Costco and Air Miles. Air Miles is by far the best deal if you have enough points. For me as a Canadian I can get a car for less than 15 dollars. When I can't get an air miles rental my second choice is Costco.
For post cruise if you have the day definitely get the car. Good time to head to Downtown Disney, the outlet mall, Ifly, Kennedy, Kelsey's for dinner, etc. We brought a car back to Budget the one time and the lady told us to keep it and enjoy it since it's a 24 hour rental anyways. The rental shuttles are all there when you get off the ship. Only thing to keep in mind is that one gas station across from the airport Suncoast charges twice as much as anyone else for gas. Better go 1 minute down the street.