Well, that really just depends on whether you like Christmas or Halloween more.... and on your tolerance for heat vs. cold.
We were the last of September into early October in 09. They had record heat that week. It was just as hot as summer. That made me nervous about trying again. That year it was still in the 90's ON Halloween. But generally the weather starts to break and offer cooler evenings SOME time in October. There's just no guarantee of that.
The parks are more done up for Christmas than for Halloween. There are a few decorations in the MK for Halloween, but nowhere else. At Christmas, all of the parks and resorts have something.
I of course like the idea of going to MNSSHP so we can dress up. But it's really no fun to plan a great costume, only to have it far to hot to ever wear.
If you ever consider Christmas though, I'd plan to complete travel by the weekend before Christmas. The resort rates get really high after that point, and the parks get far too busy for the kind of fun I want.
My ideal trip would be from 11/1 to about 11/10, covering the last MNSSP and the first
MVMCP. I'm REALLY going to do that sometime. But we have too much fun with Halloween here. While the kids are young enough to trick or treat, we'll always be home ON Halloween, and probably too tired to leave too early the next day!