The week BEFORE Thanksgiving will be a little less crowded than the first full week of December.
Some other variables to put in to your decision:
Most Christmas decorations will be up and ready that week before, but there may be a few that aren't quite going yet. You will be able to see the castle lighting and Osborne Lights for that week.
You will not be able to see the Holiday Storytellers in Epcot (Santa or variation thereof in each country) or the Candlelight Processional until after Thanksgiving.
You will also have the taping of the Christmas parade at MK that first week of Dec which can make planning a little more difficult.
One more thing is weather. It seems like something happens the week of Thanksgiving. That first full week of Dec can be anywhere from 80's to even as low as 40's at night. I haven't seen that from people reporting back from the week before.