My rental was cheaper at the Dolphin too, but only by 2 cents! I was very suprised.
My dates are 12/21/06 to 12/28/06 for an intermediate. I used an American Express code that I found on Mousesavers and a $20 coupon from BJ's, and came up with a total of $183.29. We're using Magical Express so I don't need a car from the airport, but just for the heck of it I checked it anyway and the total was $183.31. I thought it would have been much more expensive to use the airport location, but for my dates and the codes that I used, it was only 2 cents.