We've also gone several times in September, hitting every week except the last one. I honestly don't think you'll tell a difference crowd-wise because it's such a slow time of the year, and probably one of the lowest months for kids since most just started back to school. Seems like we remember seeing more kids from foreign countries than from U.S.
It's a great month for enjoying the water parks. It seems to be before they close either of them for rehab, and you can actually float around the lazy rivers without crashing into people.