Hope this link helps http://travel.nytimes.com/travel/guides/north-america/united-states/florida/when-to-go.html

Obviously, this is just my opinion. My family have been in May, September, October and last week July/first week of August. May and October were easily the best. July/August was very hot and humid with lots of mid afternoon thunderstorms and heavy rain. The heat at that time of year was draining and the parks were very busy, it wasn't unusual to queue in excess of 90 minutes for popular rides during mid afternoon. If you do go late August you may fare better from the point of view that the parks will be quieter as American schools go back mid August. For me though May or October wins hands down, temps in the mid to high 80s but without the stifling humidity.