In 2003 we ate at The Plaza Restaurant for an early dinner and were in the park already about 4 or so. That worked out really good. While they were emptying out the park of regular guests, we took advantage of short lines in Tomorrow Land and then when they opened up the rest of the park, we were able to do those rides with few lines. We had a wonderful time.
This year we ate at the Grand Floridian Cafe about 5 and got to the party about 6:30. We went on VERY few rides, it seemed totally crowded, we ended up leaving early. Both times were the first week in December in the middle of the week, so probably not a whole lot of difference in the crowds.
We also went to
MNSSHP this last with and ate in the park at Cosmic Rays for an early dinner and did the same as our first Christmas. It worked out great again.
So MY advice would be to have an early dinner between 4 and 5 at either Cosmic Rays or The Plaza and then while they're clearing out the park, spend time doing rides in Tomorrowland. You can always have snack from Main Street Bakery during the parade.
Also, we've found that the best time to do pictures is at the end of the party. There are positively NO lines, especially at the place in Adventure Land. So if you do plan on staying to the end, you could do that right before closing. Another time we did it was right after we ate that first year, but we did wait about 20 to 30 minutes.
Anyway, the reason I dont' think it's good to eat at any of the resorts, is because the resort restuarants do not open until about 5, so that's the earliest seating you can get. If you want to take full advantage of the party, I would get there around 4 and eat in the park somewhere. The Plaza Restaurant has excellent food.