There's going to be a lot of buzz (causing prices to stay high) around the Poly due to the new rooms and other construction going on, and it's possible that there may still be some form of construction going on in October.
I've stayed at Poly and Beach several times, and they are both great experiences. I recommend staying at the Beach Club again, and while you are there, take a few hours and walk around the new grounds/lobby of the Polynesian to see what you think about it. Ask to see what the new rooms look like. In my opinion, the new Poly lobby is lame compared to the fountain that used to be there. However, if you're a foodie, the Poly options are leaps and bounds better than what's offered at the Beach (and Yacht) Club.