I'm desperately hoping for my first May trip next year, so I'm glad to hear all this happy talk about May....
Choosing all the times I've been (extensive experience with June, September, October; limited experience with July, November, and yikes, New Years Eve) I'd take September. These things are always subjective and depend entirely on what you want out of your vacation.
We find we don't mind the afternoon rains of September (and they just aren't THAT bad), and we like it to feel like Florida (meaning the heat). To us, October (while a GREAT time to visit Disney, as are most months) is just a bit to iffy on the temperature. To us, if the high that day is under 70 and the low that night is below 50, it's not Florida. That doesn't happen all the time in October, but from my experience, it's always a threat (just as bad rain is a threat in September).
And we love the deals and low crowds of September, which seem to be good but not as good in October.
Gotta weigh those pluses and minuses and decide for yourself!
Pat