If I were staying offsite, I'd look for a resort restaurant for my Christmas meal. The parks (particularly MK) can become so packed on Christmas day that they close, and you won't get in even with an ADR.
One of the early stages of closing is to deny entry to offsite guests, so your chances of being locked out of some of the parks is greater than guests that are staying onsite. If your Christmas meal is at a resort, you won't have to worry about this at all.