For August weather, I'd stay at the Polynesian - it would add to the tropical theme! We loved the Poly with the tropical music, the pool with a view of the castle, the tiki torches at night, Spirit of Aloha. We really enjoyed our stay there. Ohana has a great character breakfast and a nice themed dinner. Love the lobby. Our Tahiti building room faced the castle and a quiet beach and the lagoon.
We are staying at the Grand Floridian next December just to do something different and because of the Christmas tree, gingerbread house, decorations, and holiday music (orchestra, piano player, local choirs).
The wonderful thing is both hotels are so close to each other. You can take the boat from the Grand to the Poly, and take the monorail from Poly to the Grand. You can certainly visit each resort regardless where you stay and that's what we did in 2009 and plan to do in 2010. And being on the monorail... we loved that. Fast to Magic Kingdom. Fast access to a wide variety of restaurants.