I would go with a if possible. Option B would work as well. I have to run mine more like B, but I have my fresh air passive intake @ the lower opposite side of the room, and my Filter in the upper opposing corner. The less bends in ducting, pipe, etc, you are just reducing any pressure loss. Ideally, you want that cool air too pass through your lights to cool them, from any cool air you have entering the room. If your room is just warm, you are still exhausting that hot air. Even if you don't have the most ideal air cooling system (i.e. Air conditioner, open window with cool air) the hot/warm air needs to be vented out via fan, like you indicated with your drawing.