are you going to be using a surface skimmer box? go to the "glass holes" website and check out their overflows. they are pretty nice. i would always drill the back pane. it doesn't make sense to me to drill the bottom only to have to take away valuable real estate to cover it with an obtrusive surface skimmer box.
The most asthetically pleasing IMO would be to drill a hole towards the bottom right corner of the back pane when looking at it. Usually tanks 30+ have a tempered glass bottom and those are not suggested to be drilled with conventional diamond bits, you'll just shatter the bottom if you try. I did a 29G tank the way I suggest, bottom right corner of back pane, and I built a DIY overflow box that goes for the whole height of the tank. I made the slits for the water to pass through by using a table saw and just using the width of the blade. It worked out pretty well for my goal, I'll post pics of what the tank looks like when I get the chance. But I also have another tank that is just drilled on the top right corner of the back pane and I just put a screen over the hole and it looks ok but definitely not as good as building and overflow