I've been at both of these times. My OPINION is that early December will be less crowded. Why? The Food & Wine Festival will be going on during the dates you've mentioned in late October. This event can affect the size of the early October crowds as well, but I can't say this with any authority. The festival begins earlier than normal this year, so no one can guess what will happen. If the October trip is strictly for attending
MNSSHP, I don't think the festival will affect the crowd size for the particular party you choose to attend. It is a special ticket event...only a certain number of tickets will be sold for each night of the party. That said, the closer to Halloween, the greater the chance of finding sold out parties.
If you're wanting to experience MNSSHP for the first time, I say go for it. It's a blast!! I never dreamed it would be so much fun! I personally prefer this
party over
MVMCP. Don't take that wrong, I absolutely LOVE December in WDW because of all the beautiful decor, the Candlelight Processional, the Osborne Family Lights, etc. However, if I had to choose between attending either MNSSHP or MVMCP (just the special ticket events), I'd choose MNSSHP. I just think the Halloween party is much for festive because of the "spooky transformation" that occurs just before the party begins. It's like nothing you've ever experienced before.
Oh, and as far as weather, it will be cooler in the evenings in early December than it will be in October. Keep the costumes lightweight!!! It also wouldn't hurt to have shorts and T-shirts handy for the kids to change into later in the night. (My DD always "gets tired" of her costume before the party ends.) The days, of course, are hot and humid in Oct. That's been my experience, but then again there are always those "record" years, so who really knows.