hi just come back from the states, did a long search before I left for best/cheapest car hire went for usrentacar.
cost £ 220 for 4 door compact (v.small) but they are doing FREE upgrade to intermedate car plus tank of fuel and additional driver with 'budget' from Sanford Airport.
car was of good size for 2 adults + 1 child + 1 teen and took 3 large suitcases in boot + small one in the car.
booked 3 days before leaving and had no problems, just needed emailed confirmation slip.
also anyone reading another good tip to save money, if you collect TESCO points you can redeam £ 25 worth for purple car parking ( BCP) for 2 weeks car parking at Gatwick but you must do this a few weeks in advance.
Hope this is of some help.
pineapple.