hole placement

[martym]

I'm going to drill my tank for overflows. Which placement is better on the bottom with a Durso, or on the backside?
Here is my thougths. holes in the bottom, can the water in the lower part of the overflow actually go stale, because everything is taken from the top part of the overflow. With hole in the back, I could make the overflow box shallower, therefore have a complete turn over of water in the overflow, and the boxes would take up less room. Any comments, or suggestions?
Thank you
PS the tank is 135g 6' long