Neil.

The basic idea is the heel-toe orientation of the club at impact. Which, yes, too flat means heel raised toe down will cause the ball to go right. And toe up heel down will cause the ball to go left(too upright).

However that is just one part…

If you are a 0.9 handicap then you could be playing any iron in the titleist line up. But if you are between the ap1 and ap2s then i would definitely go with the ap2's! Smaller, softer, forged. Go hit them both and see witch one you like better!