Placement of the sub is quite important, and probably more important than the type of sub you get. But to answer your specific question, I think it depends on your room. If you have a suspended floor, a downward firing sub will be felt more as it's firing directly onto the floor, instead of firing into the room. If your floor is concrete, the downward firing won't really have this benefit. Firing your sub into a wall or into a corner can improve it's output level, having the benefit of not having to drive it as hard. This is only recommended if you have solid walls.