I had a 135 with corner overflows and a 240 with a center overflow...I think I had far better turnover coming from both ends rather than the center.Water from both ends instead of the center give a better turnover.
Its really personal preference. The key is total surface area of the overflow wier. The longer the top is the more capacity it will have. Myself I prefer center overflows but I went one step further when I had LeeMar build my tank and had them offset the center overflow to the left for a different look.