I went there late september 2006, that was sweet for the crows and temperature wasn't too high (still florida high though) Prices are low as well. Beside the love bugs crawling outside your hotel door, very few guests. We never waited more than 20 minutes for a ride, and that was the first automn of Everest ! Only one day of high humidity !
I'd go back around that time because you can enjoy both water parks !
Comparing the world's 5 Magic Kingdoms on my blog : 5 Kingdoms