As MousekaMaddi says, end of Sept is HOT, early Oct is nicer... crowds are relatively light. Early Dec the weather is nice, but maybe even a little chilly some days depending on where YOU are from and what YOU are used to. We are from Montana, and I don't care how cold the locals say it is, we always find it to be plently warm in FL. Of course, we wear short-sleeves and shorts at 55° in MT, we are weird that way...
Anyhow, I think the biggest question if *I* were considering those two times would be; What do you most want to see/do? Would you most enjoy the fall decorations and the Halloween events? Or would you most enjoy the Christmas decorations and events? Would you like to do the Not So Scary Halloween Party? Or the Very Merry Christmas Party? Would you like to see Candlelight Processional? There is so much to do at either of those times... either one would be an awesome time to visit!