September is guaranteed swimming weather...because it's ridiculously hot and humid. September is easier to pack for because while you'll probably want a poncho, you won't likely need long sleeves/pants.
December is iffy swimming weather, but the flip side is it's low humidity when touring the parks. December is more variable, so you'll want to pack layers.
Both will have days MK closes early for parties, either Halloween or Christmas.
Personally, my preference would be Nov/Dec because I don't enjoy touring on the surface of the sun. However, some need the heat of Florida, and would feel cheated if they don't get their pool time because they went to Disney during a freak cold snap.