My family has been during both times. We like them both as it is hot. In early June, there are less people as some of the schools are not off yet. Based on our experience, it was less crowded but rained more in June (sometimes for 2-3 days, ALL day). In August, it was hotter and there were more people. As long as you go early, the crowds won't be a big problem. We love them both!
By this August, our house will be built (in Windermere where we can SEE MK fireworks from our yard) and we will get FL Annual Passes so we will be there a LOT (at least until we get tired of it).
Either way, it's warm enough for the water parks, swimming, and packing lightly!
Lisa