In the center you would be able to get a slower water flow, which from what I've been told is better for the fuge. I have seen some in the center and they work nice. In reality as long as you have one IMO
it will help out the tank no matter which way you run it. Also having room for a skimmer in the sump will play a role in which way u run it. Unless your skimmer is out of the sump?