Its actually AS/Onhit Kog, AD Kog, and AP kog.
AP Kog is mostly relying on his Ult to deal heavy harassment and artillery damage. He would go MID. Since his ult doesnt scale super well, he will likely be more focused on cooldown, tons of mana, and magic penetration. Rank 3 Ult deals 400 champ dmg which is pretty good when firing once per second (with no CD)!
AS/Onhit used to be stronger, but with the new metagame AD Kog now rules. AD works beter with Offensive masteries, runes, lifesteal, and crits later on. So AD gives a much better early game, despite AS/onhit techinically being better DPS midgame.