I would not worry about your second trip not being as good as the first. Every trip to Negril is irie, regardless of the time of year. If you go during the slow season, there may not be many big name musical acts but there will still be plenty of great local musicians.

Personally, I would try to avoid Jamaica in September, due to hurricane season. Too much risk of travel problems and a washed-out vacation. However, we have been to Negril twice in early November, and the weather has always been great.