You hit on my favorite times of the year.
I would say, having seen Christmastime a few times already, try October.
We normally go in October, the third week (like last year was the 13th to the 20th). Crowds are comparable to early December, weather is mild (probably somewhat warmer), and park hours are also pretty similar.
MNSSHP is a blast, and last year's Halloween/Fall decorations were beautiful; they were the most I've ever seen wdw do for that holiday/season.
I did a really long trip report on October 2007 over
here.
I can't think of any particular drawbacks. I think both Christmas and Halloween holidays are spectacular at the world. It's been a few years since we were down there for Christmastime (we were supposed to go Thanksgiving week this year, but changed it to mid-September. Next year is October again, so it will be a few more years til :xmas: )
As far as November goes, I think halloween lasts til the first week (I may be wrong), and Christmas goes up earlier and earlier, almost as early as the first week of November. Crowds are pretty low, minus Thanksgiving, which is very busy (if you want low crowds, avoid this week at all costs!)
Good luck! Have fun planning!