We've done both time frames over the last three years. This year we're actually going in October for 5 nights - 19th to 24th (Business) and in December - 11th to 18th. In my opinion, these are the two best times to visit.
Reasons why I like October:
1. Perfect weather
2. Light crowds
3. Mickey's Halloween Party
Only bad thing, the weather can turn ugly. In October 2005, we were there aand part of Hurricane Wilma. They closed MK & Epcot for most of the day and AK and MGM were closed the entire day.
Reasons I like December:
1. The weather in Orlando is always better than Cleveland in December. It might not be in the 80's, but the low to mid 70's in December sounds great to me.
2. The Christmas decorations are outstanding. Each park and hotel looks fantastic.
3. Mickey's Christmas Party.
4. The Candlelight Processional - This is a must see, really puts you in the Christmas spirit.
5. Epcot - The Christmas storytelling by the respective countries Santa's is excellent, our kids really enjoyed the stories.
6. Crowds aren't too bad until the week before Christmas.
Conclusion - I like both, but love the December trip.