I think there are too many factors (when you're going, where, does AAA have anything left, etc etc etc) to make a blanket statement about who will be the best.
I know that for our planned trip (DL not WDW), Costco was better, over AAA and Disney directly. Their prices weren't all that different from each other (actually, while getting the deposit and dates prepared, I missed on some EXCELLENT deals through AAA and Costco, and by the time I booked it was only a *good* deal), but it was the extras from each that tilted my favor in one direction or another.
When I was looking into WDW at the time that you could book
free dining (almost changed plans to FL from CA), I didn't check with AAA, but I personally noticed that the Disney specialists from whom I got a quote were EXACTLY the same as booking directly through Disneyworld.com...the difference was the amenities, they would book the ADRs, etc etc. Others might have had a different experience.
Just find out the whole list of what is included and what the benefits to you are, find out the bottom line cost, and you'll have your decision.
On the other hand, if you arent' normally a member of AAA and got a membership just for this travel, then use them. I get my membership in discounts at Tully's Coffee and in maps and guidebooks (and cheap passport photos last week) through the year, so I never feel a need to book travel through them. But others might feel differently.
