Hello fellow Nebraskan! I'll try and cut right to the chase:
Traveling in December is absolutely magical with all of the decorations and the mild weather, but the dates you have mentioned will have you visiting during one of the busiest weeks and most expensive weeks. The water parks will still be open and the water is heated, but the weather could possibly chilly to make the visit a bit uncomfortable.
A trip in late August will have you visiting with high temperatures, but the crowds will be smaller (possibly more crowded with Labor Day weekend) and the costs less expensive. Visiting the water parks during these dates will probably be lots of fun and a nice break from the heat/humidity. No extra holiday theming will be in place, but it is still wonderful!
I hope this helps to give you a quick overview of the pros and cons of the two dates. Have you priced out an estimate of the trip at the
WDW website? That may help you as well.
My fingers are crossed, too, that winter will be ending soon here in Nebraska. it has been too cold for too long! Kara