You have to look at more than the discount percentage. I've noticed lately that the minimum stay requirement for the best discount has been lower for AP discounts than GP discounts. For example, the AP discount might be 40% for days 1-3, and 45% for days 4+, and the GP discount might be 35% for days 1-4, and 40% for days 5+. So not only are you getting a slightly better discount, but if you are staying for four days or more, you would be bumped up to a higher discount percentage on the AP sooner than you would on the GP.
You always have to do the math all the way through to see which deals are going to be better for you. I would never suggest that someone buy an AP just for the room discounts, or any other benefit other than admission. Disney is not obligated to offer room discounts to anyone - they're just icing on the cake if you get them. An AP is basically discounted admission to the parks if you go more than 10 days per year.