If you want to use the bottle as a stock, then get an ASA that mounts directly on the bottom of the grip frame. That will keep your gun at its lowest profile and allow you to operate the trigger without bending your wrist around the large diameter of a tank that is mounted on a drop forward placing it directly under your grip frame. That is also the correct spacing to keep your mask off of the back of the gun when aiming. When using a drop forward, quite often the bottle is far enough forward, that you can't hold the gun tight when firing without scratching your lens on the back of the gun.
When shopping for a drop, consider an on-off type. That way you can attach a bottle and play around with the gun without gassing up the valve.
Except for the Automag in front, its usually the man behind the equipment that counts.