AP Kayle is better imo because it utilizes her abilities better, plus it scales very nicely with items like malady and nashor's. This build really reaches its full potential with Runaans, as it applies her on hit ability to several targets, which along with malady, lowers enemy armor and magic resist significantly to groups and also triples the effects of her e. Yes, Runaans works with Kayle when she activates her e, and yes, it applies her e to every target as if it were the primary target, which means each hit also has its own splash damage. In groups, this means your e hits 3 times for insane splash dmg. You will get targeted first, so get Zhonya's next, so you can stasis and survive. Swap to Sorcerer boots at the end. You'll have more than enough attack speed and the magic pen at this point will be more beneficial.

Not going to explain much about laning, I'd hope you'd know how to do that anyway.