January for sure. We went this year on those exact dates in January and the crowds were low and the weather was perfect. September is way too hot and humid and rainy.
OP it appears that the week in January will miss MLK weekend which i'm told gets crowded. We enjoy the week or two prior to President's day weekend in February.
September is supposed to be the most active hurricane month in Florida but if you're traveling from the north in the winter you could hit bad weather too on way down, both remote chances, but might want to invest in travel insurance.
Another vote for Dec. I like to be able to swim when at Disney. Also although both months are light, Sept is when many students are just going back to school, so people are less apt to take a vacation then.
I actually would probably prefer the cooler weather of your January/February dates, but I picked the September dates because you can do Mickey's Halloween Party. We have done it in September before, those exact dates actaully, and it was awesome! We dressed as pirates
I think both times are good. We prefer the cooler weather, but if you're into swimming/water parks that's certainly a consideration. As has been pointed out, the Fantasyland expansion work will be much further along during September. Depending on how often you frequent WDW, that's another major factor. Have fun deciding and planning!
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