My first choice would be CP before the party, preferably during the hour the park is closed like around 5:00p or 5:30p.
CP closes at 6:00p party nights.
I agree too. We did a 5:10pm ADR at CP on the night of our MVMCP and it was great! They have plenty of space - I've seen big parties there several times.
Personally I wouldn't eat during the party. For as much as the tickets cost and for all the Holiday shows and things to see even 5 hours never feels like enough. Buffets usually are quicker but even with an ADR at CP it could take an hour to an hour and a half to eat, especially with a larger party (i.e.waiting for everyone to finish, characters to come around, etc.). I would eat at CP with the last available seating before the party when the park closes (if you have hopper passes and can get in the park before it closes to the public). If you don't have hoppers, I would go to CM before the party. Have fun!!