You'll be at WDW during the time we were there last year.(12/19 - 01/03).
I can tell that by the 23rd, the parks are beginning to get very, very crowded. I wouldn't consider doing anything but taking the bus to Epcot.
Last year, the bus service was excellent from the WL. In fact, much better than what we experienced from OKW this past June. Just guessing, but I'm betting that they increase the number of busses and runs due to the crowds.
Whatever the reason, we never waited more than 5 minutes for a bus to anywhere. Just fighting the traffic and parking your own car would more than eat up the time you may have to wait for a bus. The return trip would have the same problems, especially if you stayed until close to closing time at Epcot. The crowds waiting to get on trams made me really glad that we took the bus!