Usually AP rates (when available) offer more of a discount than AAA. However, AAA rates are already available for Sept.
The good news is that you don't need an AAA card or an Annual Pass at the time you book your room. You only need to show them at check-in.
So, here's what I do. I book the room at the AAA rate. I keep an eye out for AP discounts and when they are released, I call to try and get the AP rate. You might try going through
Dreams Unlimited -- they will try to get you an AP rate automatically, if you request it, when they are released. That would let you not have to worry about when AP rates will be released.
If you end up only getting the AAA rate, buy an AAA card before your trip. If you get the AP rate, purchase an AP before or during your trip.
If you end up getting an AP, you may want to look into getting a Disney Dining Experience card. It costs $50 and is available only to Florida residents and AP holders. You get 20% off many sit-down restaurants and a few counter service places in WDW. You do need to have an AP voucher in hand before ordering a DDE card. Here is a link to discounts offered for AP holders and DDE card holders:
http://allearsnet.com/pl/discounts.htm
I hope this helps!
EthansMom (and Molly's too)