RE: 3D Foamy Sukhoi, my experience.
This way the battery is much more centered on the plane, both horizontally and vertically. I have it strapped up against the side fuse rails. I cut a piece out of the fuse to slide the battery through. It really does help distribute the weight much more evenly throughout the plane. With this mounting location, the battery is almost perfectly inline with the prop shaft and the wing - right at the center of thrust. It really seems to help in a hover.
This picture was taken before I got the servo extensions delivered. I've cleaned up the wires some since. I'll try to get another post with everything hooked up, but at least you can see the battery location here.