Unless you plan to spend time off site, my advice is not to rent a car. As already mentioned, Disney will add additional buses when it is warranted.
I would not want to drive in the traffic during Christmas time - too many cars with too many people who do not know where they are going! Even if I have to wait, I'd rather let a "professional" deal with the crowded roads - way less stressful. I'm on vacation!
Best wishes -