We had the same choice as you and decided to do it on Christmas Day. We did the dinner package, which I think is an absolute must if you're going to go to the CP on either day. We had dinner at Le Cellier and then went to the 8pm show. It was a wonderful way to end the day.
The reasoning behind our decision was that we knew the MK would be terribly crowded on Christmas Day. We wanted to spend one of the 2 days there and decided it wouldn't be as bad on Christmas Eve and that Epcot would be much less crowded on Christmas Day than the MK. So, we had Christmas Eve dinner at the castle, then did Epcot and the CP on Christmas Day.
Either day would be nice, but be sure and reserve as early as possible!