Hi Avery,
Both the
Tomorrowland Terrace Fireworks Dessert Party and
Mickey's Very Merry Christmas Party (MVMCP) are incredible!
Personally, I would skip the Dessert Party and focus my attention on all of the special events going on for MVMCP. There is a special parade, fireworks, dance parties, character meet & greets and shows that take place during MVMCP. Because you have to purchase a special event ticket for MVMCP, I'd want to full advantage of those opportunities. Its also great to enjoy the attractions with lower wait times.
The Dessert Party is fabulous and offers one of the best views of the fireworks inside the Magic Kingdom. It begins an hour prior to the fireworks and you get to enjoy some amazing desserts while you wait. While on most occasions, its great to have your own space, there is just too much going on during MVMCP that I would not want to miss out on.
If you are able to visit the Magic Kingdom another evening, perhaps you could get reservations for the Dessert Party that evening. You could also save it for another trip to Disney World. The Dessert Party is offered many more times throughout the year while MVMCP is only offered a few select evenings during the holiday season.
Wishing you minnie magical memories :-) Allison