I had two corner overflows on my old 125 and HATED them. They were small... 6"w x 6"d x 24"h +/-... but when you look at the tank from the side you get a dead spot where you can't see anything behind it. It's ok if you look from the front only but my tank is against one wall on the right and I blocked my view. Great place for fish to hide though!
I now have a single center overflow on my 140 (same size, just a bit taller) and now when I look at the tank it's wide & clear... can see all the way down the middle of the tank. Also made plumbing to my sump easier as all the tubes were centralized rather than coming from both sides of the tank.
Just my experience though.