I'm certainly no expert but from my experience, the weather seems to be quite unpredictable during December.
I would say to go from the 3rd - 10th, becuase the lines will be (somewhat) shorter, based on my experience. We went from Dec 4th - 13th, 2004, and as silly as it may sound, I felt as though the crowds were seemingly larger towards the last 3 - 4 days of our trip. Most schools let out in mid December. December 16th, 2005 for our school district.
Remember when school lets out crowds swell at WDW. And a few days before and after too. I would definitely say 3rd - 10th.
Have a great time whatever you may decide.
Oh, and the absolute best tip I got here before we went was to pack some hats and gloves for the kids. We did not need them for most of the trip (83 degrees many days). But on the couple of nights when we needed them, we were the envy of many people at the parks. We picked up some really really cool Mickey hats and gloves for all three kids at Wal-Mart and each hat & glove set was less than $6!