When we lived in FL and had Resident Passes, we would go really often in both months. Between the two, September was generally less crowded...but often much warmer and humid than October, especially in the evenings. October evenings could be really, really pleasant. So, if the weather is not an issue for you (and you are in Hurricane Season in either month, so that's a non-issue...) than I'd say as far as crowds go, September is a little better. We'd always drop our kids off at school and drive there for the day until it was time to go pick them up. Very low crowds during the week in September.