Well based on the data, his S1 scales off of 100% attack and 10% HP. That is to say, using his awakened, max level stats of around 1000 attack and 7500 HP, every 10% increase in attack will give him 100 extra "attack", while every 10% increase in HP will give him 75 extra "attack". This is of course assuming that the equipment % stats are based off of his base stats. With that in mind, he WILL get a higher "attack" stat by building attack, but it's just so much more efficient to build HP on him. You only get a -25% bonus attack penalty for building TANKY AF as opposed to a pure glass cannon. His ult (with a scaling of 30% HP) will definitely hit harder with an HP build. Vigor will scale slightly better if you build attack though.