IMO Hands down Drilled, With a overflow box style if you lose your siphon on a power outage there is a possibility of the box not starting back up and your tank overflowing so you are relying on something mechanical in order to keep things goin, whereas with a drilled tank you are relying on gravity. Mechanical things will fail. It is not a matter of if it is a matter of when. Gravity will never let you down.