Both times of year are amazing to visit, and have lower crowds. December from my experience is hit or miss with weather. I went the first week of December in 2006, and some days we did use the pool, other days it was a little cooler. It just depends on what happens the week you are there.
7-10 days is a good amount of time, depending on how much time you can get off of work. If there's holiday activities going on, you'll want to do the parks, see the holiday special things, and have some down time.
MVMCP is a separate ticketed event, taking place in the evening at The Magic Kingdom around holiday time. You'll have to purchase a separate ticket, besides the park ticket. It's really fun! They make it "snow" on main street. All the characters are in their holiday gear. There's numerous stations where you can get free cocoa & cookies. I think it's worth doing.
I've never done MNSSHP, but it is also a separate ticketed event at the Magic Kingdom and is around Halloween time on certain dates. I think there's a special parade/fireworks display, and a trick or treating type thing. Someone who has been there will be able to tell you more. I was at Disneyland for Halloween this year, and if it's anything like that, it's also a very unique time to visit. Decorations were fantastic, and we had a lot of fun! October will most likely be warmer than December.
