Wayward Son
Contributor
Where you put it may well depend on what kind of diving you do. I mostly spear fish, so I need to be able to load the gun. A bottle slung under my arm is going to be a PITA in this regard. Inverted & mounted to my primary tank, with the 2nd clipped to a D ring on my right shoulder strap, it's totally out of the way.
YMMV
YMMV