Been there during both times. I would choose December, esp. if it's early during the very slow time. While September is hot, it is also hurricane season, so your trip could be completely rained out. In December, you may have some cooler nights 50ish, it is usually wonderful during the day 70s-80s. Still pretty warm to swim. Plus you get the added bonus of the possibility of e-ticket nights, not something available in September it seems (the past couple of years). PLUS, PLUS, you get to enjoy the holidays decorations and shows. Both are value seasons (if early Dec.).