We did the third week in August last year and then again in mid-Dec 2003. August was hot/humid/really rainy. December was chilly. We knew it would rain everyday in August but it was torrential and lasted about 6 hrs. But, I believe that was somewhat unusual. So, we are going back again in August this year. I don't mind the heat/humidity. There is a lot of air conditioning you can get into. The pools are wonderfull. The crowds weren't horrible. The only time we found the parks to be really crowded was when it rained for hours and there was lightning. Then the attractions in the water shut down and everyone went to the attractions that were still open!!
In December, we found it to be nice. The days were around 70 degrees and the nights were cool. By the time we left, it was downright cold!!! I would rather be hot. So, we are going for Thanksgiving this year. Still be able to get in the holiday mood but not be so darn cold.