I've been there in July and in late August. Both were very hot, very humid and it rained just about every single afternoon for a short period.
The advantage of going in August is the lighter crowds and it's Value season at most of the resorts. The trade off is that the parks are not open as late. Fantasmic! will only have one show and they may cut back on other nighttime shows as well.
My vote would still be for August. But if you look at my counter, I'm going in July this year.