I don't think you can go above 10 days, so the question is whether a 10+5 day is better than an AP. Looks like the gate price of a 10 day+5 day is 449.43.
Are you really going to use 15 days at the parks? Maybe you could get a 10 day MYW with water parks and more. That way you could visit the water parks/Disney Quest/PLeasure Island. Those are $283.29 and includes 6 visits to the water parks and other minor attractions.
Marsha