We're going down between Dec 23 and Jan 1, so I've priced rental cars for a period somewhat similar to you. I've generally found the non-airport locations to be a little cheaper. As for the best place with the lowest rate, it really depends on what size you want to rent. I would suggest going to mousesavers.com and check out the section on Rental Cars. They provide a lot of codes you can use to get discounts (although not many extend through your dates).
For what it's worth, we've reserved a full size for 12/13 to 12/31 from the Dollar located at the Grosvernor Resort near DTD for about $230. Hopefully, we'll be able to find something cheaper as the dates draw closer.