Mnsshp & Mvmcp
I have been to both the Halloween Party and the Christmas Party.
The Halloween Party was great - an awesome parade, HalloWishes was fantastic, and free candy is a nice touch. There are shorter waits for lines, and for all of September and most of October it is more than warm enough to stay in MK until past midnight. There are special pins, shirts, and photo ops during MNSSHP. It is fun to see all the characters in costume - I have pictures of Eeyore as a mummy and Piglet as a flower! The only downside I found during the Halloween Party was the amount of children and teens barreling through groups in search of free candy.
The Christmas Party was wonderful - it snows on Main Street!! The Christmas Tree is beautiful, and they serve all the hot cocoa and snickerdoodles you want. The characters will wear scarves and hats, and Donald has his own 'tree lot' - also, this is the ONLY time I've ever seen Scrooge McDuck! There are shorter lines for rides, but it is colder. We went in the beginning of December, and it was in the mid 50's after 9pm. I liked it, especially being from New York, it was nice to be bundled up, wandering around with (fake) snow falling around me, sipping hot cocoa. There is also a special parade and fireworks show, and the Holiday Wishes is very nice. There are also special pins and shirts, etc. during this party.
Personally, I like the atmosphere of the Christmas Party better; it felt like not everyone was rushing around for something (ie: candy.). BUT I like the photo ops at the Halloween Party better. I will be attending both this year. If you have the money and the time, I would definitely go once. It's a special time of the year, and it's something not everyone gets to do.