It will actually be less expensive to stay on property the week before Christmas than it will the week before Thanksgiving. That may seem a little odd but the reason is that Disney has "seasons" for their hotel rates.
The 11/19-11/26 stay falls in "Regular Season", while the 12/17-12/24 stay actually begins in "Value Season" and extends into "Holiday Season". The beauty of it is that if your stay begins in value season, you pay the Value Season rate for
all nights of your stay, even if they fall in Holiday Season.
Another consideration would be the extra holiday events that do not take place prior to Thanksgiving. You have the resorts and parks all decorated for Christmas,
MVMCP, the Candlelight Processional, Osborne Family Lights and Christmas shows in every park. It's a magical time of year to visit WDW and if you have the chance, that's when I would recommend doing it.
You might be concerned about crowds but with planning and patience, you're sure to have an enjoyable time.