Wow! That's a tough choice.
Sept pros:
warm enough to swim
free dining
very low crowds
Sept cons:
possible hurricane or at least a bad thunderstorm
can be very hot
Dec pros:
weather is cooler
xmas decorations
Dec cons:
more crowded
can be very, very cold (especially at night)
too cold to swim
if it does rain all day, you're both wet AND cold (it was like that last year)
I enjoyed our Sept trip much more than our 2 Dec trips because the crowds were practically non-existent. We had wonderful weather in Sept (yes, it was hot in the afternoon, but we swam then). Last Dec it rained and we froze at night.
The main problem with both times is you just can't depend on the weather to be beautiful. In December, you can count on there being no hurricanes, but you can't count on no rain. In September, you can count on it being warm enough for swimming and no winter coats, but once again, you can't count on no rain. At least in Sept if you get rained on, you'll dry off quickly...in December you walk around the parks wet and cold.
HTH.
