Scion was my main for a long time, but I played him as tank.
That being said, I think AP is the best. For a couple reasons. There are many more AP + health items...ok 2, but RoA stacks nicely, Also the AP ratio for scions abilties are 1:1 which is op.
AD is awesome, but usually you get focus'd, if you can have a great early game and get some AD and then be workin towards frozen mallet its baller. Also AD Scion is probably the better late game build, because you dont need attack speed if you have level 3 ultimate.