Christmas is on a Tuesday this year - so you'd be arriving on the Friday before that, correct?
I definitely vote for Christmas weekend. Yes, it will be crowded but, as Malcon10t said, the crowds may be
ever so slightly lower. There is a chance that some folks will stay home or go elsewhere to visit family for the actual holiday, but after Christmas and up to New Year's lots more people are available to visit DLR, armed with gift money to spend!
Plus - and this would be a big determining factor for me because I am a huge fan of Christmas time and would want to soak up as much as I can at
Disneyland Resort - if you go while it is still before Christmas, you will get to experience extra little things. For example - Santa is at the 3 Disney hotels (there is a great photo spot with Santa by the large Christmas tree at the Grand Californian) and Dickens carolers perform at the hotels as well. The hotel Santas and the carolers clear out of the hotels sometime after Christmas and before New Year's rolls around.
You will also get to see the cute Christmas Eve Goodnight with the characters (some of whom are dressed in pajamas) in Town Square on 12/24. (Malcon10t can probably explain a bit better about that, as I'm sure she has seen it many times.) You may get to enjoy some special menu items at a few restaurants that will be there for Christmas and not for New Year's weekend.
The bottom line is, unless you just really want to ring in the New Year at Disneyland Resort (and many people do), I would stick with Christmas weekend because it will keep your holiday options open a bit more.