Depends on what is important to you. If longer hours are more important, July would be your best bet. If you are hoping for lower crowds, August will be at least slightly better for that. Schools in the south, Florida included, go back to school pretty early, so crowds do go down a bit by mid August. For reference, the major school system closest to Disney starts August 10 this year. Lately, there seems to be less variation in park hours regardless of season. This year it looks like only a difference of an hour, maybe two, between peak summer and mid August into September. It seems that gone are the days of midnight MK closings during peak summer season. Weather is about the same between the two. Hot, humid, daily chance of scattered thunderstorms. Personally, I'd go with August.