Elismom,
Whether you go to MK on Christmas depends on several factors: are you there long enough to skip a day in the parks, do you want to experience the "magic" of being in MK on Christmas day, and can you handle LARGE crowds and the fact that you will not be able to do many rides that day.
If you want to go MK that day you should get there early as it typically closes to new guests at around 10:00 am. It will reopen in the afternoon, so you should still be able to go to dinner at CRT if you don't go in the am.
The other parks are also very crowded, but not as bad as MK. I think Epcot is the best park if you want to avoid the crowds. Because of its size it seems to handle the large crowds better. Also, many people feel it isn;t as "kid friendly" as the other parks so they don't go on Christmas Day (the ultimate family day).
Whatever park you go to will be crowded. Go early and use Fast Pass if you want to go on rides. If it gets too crowded leave and return around 5:00 pm. Go and resort hop or hang out at the pool.
Have fun!!