As always, when there are 2 choices... two sets of opinions will follow! It comes down to personal preference. January seems to have more rehabs and closings, and there is only one water park open, and only on days that are deemed warm enough at that.
That said, I would pick January hands down. I don't mind a coat or jacket but foundthe heat and humidity nearly killed most of us when we went in Sept. Our strategy each day centered around heat survival, not crowds or anything else. And it rained every single day, and not just cloudbursts, either. Torrential rains, and t-storms, for more hours than not per day. I hear that we hit an extremely wet spell... 13 days of it... but that was not enough to ever lure me back in that timeframe.
I know others want heat when travelling to Florida, particularly when coming fro winter at home, but as long as I don't have to shovel, I feel I have escaped the winter.
This was our experience as well. Heat survival LOL!!