Get BOTH!!! Any packaged booked via AAA will include any available AAA discounts (there's a standard discount all year round), however if you pay with your Disney Visa, you get the 6 moths deferred payment option.
In addition, if a Discount code (be it a general one or a specific one for Disney Visa holders) comes out, your AAA rep can apply them to your reservation or make a new one.
Note that discounts are NOT stackables. That is, if a general code comes out, you'll pay EXACTLY the same as booking directly with Disney than thru AAA.
Also, booking with AAA you can opt to get insurance thru them instead of Disney's, which is a little bit cheaper and a bit more flexible.
My .02...