Aesthetically and (theoretically) practically, I prefer the top/forearm mount. As cool and cinematic as the thought of grabbing an enemy's head and melting a hole in his chest as he dangles helplessly in the air is, I figure if a dread already has you by the face, you're DONE, with or without the extra point blank shot.
I wouldn't worry too much about center of gravity. It's a small piece, relative to the whole model, and the character it's attached to is a stompy death robot housing a former stompy supersoldier. In either sense, that degree of bulk and weight is a mere pittance. Underslung made more sense on the old 'elephant foot' style of battering ram
DCCWs that mostly moved in a single plane, but it just looks like it would cause too much interference with the hand and arm movement, here. That's my take, for what it's worth.