January vs August

January's weather can vary from quite pleasant to quite cold.  We're in the 60s today and will likely go into the mid 70s tomorrow.  However, for much of this week, we will have highs in the 50s with some nights going into the 30s.  Temperatures will again moderate later this week to highs in the 60s.  With highs in the 60s, most days will be quite pleasant but evenings and early mornings will be quite cold.  Crowds can be variable in January as well.

August is generally very hot and humid with highs many days into the 90s and feeling as though it over 100.  Crowds begin to moderate somewhat by the end of the month but it is still quite busy during the first part of the month.  August can also be somewhat rainy with thunderstorms not uncommon.  Though hurricanes are fairly rare in August, it still falls within hurricane season so a storm can't be entirely ruled out.

We've been solid late August people for years and quite enjoy it, since crowds (and prices) are a bit lower than at other times of the year. But, it is HOT, like surface of the sun hot and humid. We've learned to manage (lots of walking through stores and indoor areas where possible), drinking a ton and making sure to alternate indoor and outdoor attractions. But, all it takes is the walk from your resort room to the car or bus and you're sweating buckets. Those swim breaks are much needed and much appreciated.

We did a January trip once and it was fun to see the end of the Christmas decorations, and it was nice to not being sweating every minute of the trip. But, it was rather chilly for swimming and we spent much less time in the pool or hanging around the resort.

We have been at different times throughout the year many times. As has been stated, August is hot. Highs in the mid to upper 90's with high humidity and typically afternoon thunderstorms. January is all over the place. We have been and it be as high as 82 to 85 for a couple days and then a front come through and the highs be in the mid 50's. I have never so much as dipped a toe in a pool at WDW but January is not a good time to go if you want to get in the pool. 

With all that said, I prefer going in the Winter months due to the milder weather and typically lower crowd levels. The trade off is, the park hours are shorter and they tend to close earlier. I would much rather them stay open a little later to give us more park time but it is what it is I guess.