Personal preference, maybe with a bit of ergonomics reasoning thrown into the mix.
I use a handtorch in a light sock as my primary light. When I don't need it I clip it off to my right D-ring. All my temporary clips go on the right D-ring in fact, with the exception of my fingerspool double ender, since I deploy w/ my right hand.
I use my left D-ring rarely, so I keep my backup there.
I can imagine that if you had stage bottles on your left then it might make more sense to strap your only backup to the right.
I use a handtorch in a light sock as my primary light. When I don't need it I clip it off to my right D-ring. All my temporary clips go on the right D-ring in fact, with the exception of my fingerspool double ender, since I deploy w/ my right hand.
I use my left D-ring rarely, so I keep my backup there.
I can imagine that if you had stage bottles on your left then it might make more sense to strap your only backup to the right.
