Hi, Kathy! I absolutely love both parties, but I polled the children in my house (ages five, nine, and 19), and hands down, the party winner is Mickey's Not So Scary Halloween party.
My gang loves dressing up in their costumes, trick or treating through the Magic Kingdom, and they absolutely adore the Halloween parade. It's so much fun to see some of their favorite characters in Halloween costumes. We usually watch the parade twice the night of the party, because no one wants to think they missed something!
While Mickey's Very Merry Christmas party is equally as much fun, with seemingly shorter waits for the attractions and a wonderful parade, the girls really enjoy the trick or treating. Everyone seems to get into the spirit of things, and even the adults that don't dress in costume usually have some sort of Halloween shirt or vest or something festive. It's a great time for everyone.
It would be a tough choice if I was making it alone, but the kid input is definitely Mickey's Not So Scary Halloween Party.
I hope that helps with your planning!