AP discount is usually around 5% better than the GP discount. As others have mentioned the Disney Visa often gets you access to the General Public Discount early. We book via the Disney Visa discount (ie the GP discount but early thanks to the visa) and then when the AP discount comes out we apply that to the room (so we've got the room booked at a discount just in case the AP discount doesn't apply or doesn't have availability for our dates/ room)