Thing is its only a 150w bulb, its not kicking out loads of light and its in a reflector so can only light from one side. If it were bare then you could plonk it down the middle and get equal light in all directions.
Multiply that with the fact your mainlining and it would seem more logical to go horizontal with both lights.
A proper mainlined plant will let more light in from above than it will from the side.