Are the crowds and weather the only important things for you?
Crowdwise, I think Nov/Dec will be slightly less crowded. December is value season; October is regular.
Weather...October probably has a slight advantage. It's most likely warmer on average, but at the tail end of hurricane season. December weather can be erratic. It can be beautiful (like it was the week of Dec 4-11 when we were there) or frigid for FL like it was the following week. In the end, weather probably is a toss up.
Now, do you prefer any of the goings-on at those times?
October will have the Food & Wine Fest at Epcot and then there is the opportunity for
MNSSHP at MK as well.
Nov/Dec will have the Christmas decorations and activities in full swing at Epcot and I imagine you may be able to take advantage of one of two
MVMCP dates during your time.
I have yet to be at WDW in October - although that is the plan in 2005. My b-day is October 30 and since my family has been to WDW on everyone else's b-day, DH says we need to go over mine now. I am excited about F&W Fest and MNSSHP.
But I do feel sad that I am missing out on the Xmas stuff because we have been able to see all (or some) almost every year since 2000. I love the Xmas stuff a lot so I think I will miss it. BUT I still think October will be fun.
I guess my feeling is you can't make a wrong choice here which is why I didn't vote for one or the other.