Well everyones going to have a different opinion! LOL! Neither is wrong but what is right for you and your family! But... Since you have asked! LOL! If I had a 5th wheeler & annual passes for the water park I would go before my passes ran out! Of course you are getting an answer from someone that usually goes in July! LOL! In July we do tend to make plans or reservations to eat in the afternoon somewhere cool, see an inside show to rest from the heat, return back to the camper for a swim or lunch, and return in the evening when its cooler. We take our water fans & water etc & use our magic hours some.
One thing I did not like in the fall was having to wear a jacket some mornings, & that the temperature of the pool was cold. I heard others complaining about the pool so I don't think it was just me! LOL! We didn't camp we stayed at All Stars Sports last fall 2012. I heard one family complaining to the lifeguard that they thought the pool was supposed to be heated. They said that Disney would not turn on the heaters untill a couple of more weeks which would have made it about first of November if they were correct. But I don't know if that was just at that value resort or all of them including Fort Wilderness.
Even though I have went on vacation a few Springs and Falls I just personally like a summer vacation the best! If I could have my way though, my preferred time would be first week or two in June.