I'm actually going during both of these weeks! Haha, for the reason of having lower crowds mostly. If I had to pick one or the other, it would be close, but I think I would go with January because I've seen the crowds are even lower then. I don't celebrate Christmas, but I like the additional EPCOT fireworks, but don't like that they are removing the Star Wars fireworks for Christmas fireworks during my trip. So that's a win/loss there. Neither time is one which you would want to get in the pool either unless there's a heat wave, that's a net neutral.