Any disney park you are going to be @ capacity. Magic Kingdom will be about 70,000 ppl and uncomfortable .... (i will never go there on christmas )
Sea World can move ppl through more cuz they add a lot of shows and events. There are also special shows just for holidays. The show schedule is not out yet because they are debuting some new shows and havent decided how many they are doing. There will be two clyde and seymore shows , two shamu shows, an ice skating show, plus all the normal shows. If your children are younger i think there will be sesame street characters too. You can get fastpass callled quick queue for the big attractions (manta kracken and stuff) but the park is mostly shows. Waits in happyt harbor will be up to an hr probably (sry dont know ages of kids) Sea world will be more relaxing in my opinion, and the different shows later in day. I do not know if they close at a certain capacity ... but i hope that helped