Both are going to be crazy crowded and sell out quickly. I would say do the 30th and then do the trick o treating and stuff at the resorts on the 31st itself since you are there for both days. That way you get the double dose of Halloween that isn't possible if you do the 31st event.
We will be there the same days. We are going to MNSSHP on the 30th and going to Epcot for Food & Wine on the 31st. Our thought is that both parties will be busy, but Epcot will be less busy on Monday rather than Sunday so we can have a less busy day at Food & Wine.