September is HOT in Florida. So if you like HOT, it's not bad.
Hurricane season is very unpredictable. We made an emergency departure in 2004.
Crowds are billed as "low", but heck, it's DISNEY....there will always be people around.
Early February, it can be cooler, but if you are going with kids, they are less likely to be concerned with cool as they are with hot. If you go around President's Day you will spend most of your time waiting, but heck, it's DISNEY ....it's worth the wait!
I have found that you can do Disney for less $$ in September between Disney specials and airfare.
My opinion - I prefer Disney in the first 6 months of the year!