Depends on what you want.
Pros for early December:
-Low crowd levels
-Cheaper hotels, usually some package deals or specials going on also
Cons for early December:
-Short park hours, some days might be even shorter due to Cast Member Christmas parties
-While there are nightly fireworks, Fantasmic and the electrical parade don't start nightly until later
-Not all rides might be running due to refurbs that might be going on during "off season"
Pros to late December:
-Long park hours (8AM-12AM every day and 10AM-9/10PM for DCA)
-Every ride should be up and running
-Entertainment (parades, fantasmic, etc.) will be going in full swing
Cons to late December:
-The biggest con is that the parks are SUPER crowded that last week of December and DL often close due to capacity. This means wall to wall crowds often and long lines almost all day
-Everything is more expensive during this time
So, pick and choose your dates carefully. IMO, the best time to go is just before Christmas. The park hours have just gotten longer, the rides are all running, and so are the shows, BUT the HUGE post-Christmas crowds haven't come yet. I've been twice at this time (and going again this year) and have had great success in accomplishing a lot with a lot of planning.