The resorts will be decorated before Thanksgiving - I specifically remember pulling into Coronado Springs the Wed night before Turkey day a few years ago and it looked great
I also echo the same sentiments as PPs.
Wilderness Lodge - either CCV or BRV - I go one of the first 2 weeks of December, and this resort's decor was a big factor(BRV) - Copper creek is in the main building, so you will get a little more of the Christmas vibe, but Boulder Ridge is right there. I have taken my kindle into the main lobby late at night and read in front of the Christmas Tree, it was so nice.
Grand Floridian - 2nd choice - I bought
DVC with the intent of using it for the Christmas trip because it also Rocks at Christmas! Problem is - it is very hard to get, particularly studios. Not sure what type of room you are planning on, but if it is a studio, as a renter, I would say there is 0 chance of getting a VGF studio at that time. I own there and have so much trouble getting one at 11 months, I barely even try, I go with Boulder Ridge. That is a ressie that basically has to be walked.
Here is where I diverge -Beach Club. I think it is a must see, the carousel is cool, and walking over to the Yacht Club for the Christmas Village is really cool too, but as far as Resorts at Christmas time - I have to put Animal Kingdom Lodge (Jambo House) ahead of it. Although neither are bad choices.
Board walk - Honorable mention.
Worst - (and its my fav resort in general) Polynesian - totally underwhelming at Christmas compared to other resorts.
More notes since it is your first Christmas Trip:
In past years there have been Christmas Tree pass along programs going at various resorts, not sure if they still go on, I haven't participated in a few years.
If you do not rent DVC, I would vote for Coronado Springs as best Moderate at Christmas
Rent a golf cart and drive around Fort Wilderness to see the decorations!