I'd go in September if your not a crowds person. If you really feel like you need to be there during Christmas for decorations and getting into the Christmas spirit; then going in December is a must.
The first few weeks of December the crowds are not very bad at all. During the last 2 weeks it's a mad house.
If you go in September the weather will be warmer and going to the water parks and swimming in the resort pool are options. In December it is a crap-shoot. Some days may be warm enough for swimming and others may not. How important are water activities during your trip?
I guess I would base my decision on those factors.