Just from experience in WDW during those times… it can be very hot, but you can also get some cold evenings. It is kind of the pack for all weather occasions
Personally, I would pick May because you have a better chance of good weather. September is still going into hurricane season. Camping would be no fun in a hurricane
May is great because you have Epcot's Flower and Garden festival going on til May 15; then Star War's Weekends begins at Disney Studios on May 20. Weather is great both months. Some folks might say it's a little hot, but it's great for this native Floridian.
Based on my experience, September (except Labor Day weekend) has lighter crowds...not enough to make me change my plans if I really wanted to go in May, but still lighter.
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