DW and I went over Christmas time in 2002. Before 12/25 the parks are not too crowded and you can get a lot done. Beginning 12/25 they are mobbed. That's not to say you shouldn't go or that you won't have a good time.
We did Epcot on 12/25. Since the park is so big and spread out it seems to handle the large crowds better. Also, many people don't think Epcot is a good park for kids. They'll either not go to Epcot at all or only go at the end of the trip (and most people begin their XMas vacation on or after 12/25).
MK will be wall to wall people and will probably close to new guests by mid-morning. Even if you get in you can't expect to go on many attractions. For a short trip like yours, stay away from MK on 12/25.
With that in mind, I'd recommend MK on Xmas Eve. Actually, if you are arriving earlier in the week I'd do MK as early in the week as possible.
Whatever park you go to during that week, make sure you get there early--right at opening. In 2002 they (quietly) announced that the parks would be opening one hour earlier than normal (including the EMH park). Check the opening times when you get there.
Make sure you make the most efficient use of FP. It really helps. FPs for TT and MS will go early so get them first.
Also, make sure you make PS's ahead of time for any sit down restaurants. If you don't you'll wait hours for lunch or dinner (or will only be able to do counter service).