Centered is fine and the light may be a bit stronger on the 400w side but all the light is being pushed downward and spreading outward so you shouldn't that much of a difference in light. It just might be a little hotter on the 400w side.
I would put them both on top bro cause side lighting helps but a 250 HPS might be a bit too much and you might start burning shit!!! Put them up top and watch the buds grow big!!!