Hi, Belinda! My family absolutely enjoys Mickey's Very Merry Christmas Party, and it's a wonderful event. But, I have to tell you, hands down, my absolute favorite thing at Walt Disney World is the Candlelight Processional. I cannot think of a time when I haven't been moved to tears while enjoying the performance of the fabulous choir and the celebrity narration of the story of the Nativity. It is the moment during the holiday season when I fully recognize and remember the meaning of the Christmas spirit, and it is an event that I would never miss.
You don't say if you have young children, so I will warn you that for some younger children, this may be a bit hard to sit through. I've always planned on doing the last show so that my youngest child was worn out and ready for bed. They've usually just fallen asleep in my lap (which can make it tough to stand for the Hallelujah Chorus!). The show is about 45 minutes long, and in addition to the narration, the music can best be described to a child in the words of one of my children "This is like going to Church!" You'll have to decide if that's something your family can handle.
If it were up to my family, we would choose the Candlelight Processional every time. The party is great, and if that is an option that better suits your family, you can opt to purchase one less day on your theme park passes and use the day of the party to hit the pool or do some shopping, then spend the money you would have used on a park ticket that day to purchase your Party passes.
I hope, whichever you choose, your family has a wonderful vacation!