We've been in October several times, and in December once, and we've done the Halloween party (3 times) and the Christmas party. We also went this year in January, which, weather-wise, is similar to December.
It really depends on what part of each month you would go in. If you have to go after December 15 (due to school or work schedules), I'd say go in October. Nothing in the world is more beautiful than Disney World at Christmas, but the crowds there during the school holidays are insane. The year we went in December we had great weather - no rain, sweaters during the day and light coats in the evening - but when we went this year in January, we stumbled upon the coldest weather they've had in over a decade. It was unpleasantly cold for the first 3 days we were there.
As I said, the park is gorgeous in December, but it's not too shabby in October either, and I think if I had to choose between the two parties, I'd choose the Halloween party. It's just so much fun to take the kids to the trick or treat stations between rides - and to get the kids dressed up in costumes! The end of October is a lot busier than early October, but also cooler. We usually go in early October for our anniversary, and the crowds are minimal. Weather-wise, maybe we've just been lucky (but as I said, we've been several times during October), but it's usually cool enough in the mornings and evenings that I have to wear a long sleeve shirt. During the day it's been warm enough for shorts. I will say that it's never been warm enough for us to swim in October (or December, for that matter), but that may be because my husband and I don't like to swim in even slightly cool water.
Edited to add: Sorry, I didn't see your last sentence till now. It it's between the LAST week in Oct vs. the FIRST week in Dec, I'd go with the first week in December, if for no other reason than the lower crowds.
I'd be happy to answer any other questions you have. Until recently I was a Disney travel agent, and talking Disney is just about my favorite thing to do. :-)