We always rent a car...but there are only 3 of us (2A + 1 5yo) so we can get away with a mid-size. Usually Alamo has some good rates/discounts/coupons - check over at mousesavers.
For us, I like being able to get to/from the airport myself - especially on the way home I just feel like DME makes you leave so early! On our last day we can check out, send our bags via the airline check-in at the resort, then head over to a park in the car and leave right from the park about 2 hours before our flight without having to go back to the resort. And we also make a grocery run when we first arrive for water/snacks and toiletries so we don't have to pack them or ship them. And if we have an early AM breakfast ADR, I don't have to stress about how we're going to get there. Then again, lots of people feel the rental cars are completely un-necessary and do fine without.
I'd say check the rates and see how much it will be...and keep checking and modify/cancel your car reservation as many times as you need to up until right before you leave to make sure you get the best price, then decide if it's worth it. In Dec '07 and May '08 our mid-size was just under $100 for the week. Oct '08 it was a little higher, but maybe like $120-ish.