With a pony strapped to the side of a single tank, a slight roll toward the pony would cause a greater deviation in weight distribution with respect to your roll axis, creating a destabilizing feedback loop. On the other hand, with a slung pony the same slight roll toward the pony would still move the pony's weight further from your roll axis, but it would try to pull you back toward level, creating a *stabilizing* feedback loop. (If this is hard to see, I can diagram it or something -- just ask.)
Either way, you may have to slightly redistribute weight to balance any torque from the weight of the pony, but that is static balancing not dynamic stability. With a back-mounted pony, geometry dictates that the weight above the roll axis decreases roll stability. With a slung pony, the weight of the pony increases roll stability. In fact, with your weight properly balanced, the more weight you sling, the more stable you are.
(It's just a wetter version of balancing a yardstick. Balancing it upright on your palm is precarious; dangling it like a pendulum from your finger is stable.)
