Are you going to be with kids? And if so, how old are they? The Xmas party is not cheap (we spent $400 on tickets for 2 adults and 2 kids!), so if you're going to be there all day with kids and stay through the night, will they be too tired? We are doing
MVMCP this year on a night where we are not going to the park during the day. Is it your only chance at doing an MVMCP? You can do it on a rest day too! We went at Christmas time last year and we didn't do the party because I didn't think my kids could handle it. I really wanted to do it so we're going back this year for an MVMCP and another full MK day but planning rest days around the party.
If you decide to not do it and just do your CRT reservation, it shouldn't interfere at all with the park closing.