Butt stock shell holders are, in my never to be humble opinion, a bad idea in general. In the unlikely event that you should ever be in a real fight with your gun, chances are pretty good that you might have to fire the weapon from your weak side. It would be extremely unpleasant with a face full of shotgun shells.
Clearly, not everyone shares this viewpoint, but that's what makes the world go 'round.
It's your money, your gun, your home, so do it your way. I'm just sharing what I think.
BTW, #4 buck or 00 buck would also be my choice also. You need to pattern both in your gun. The S&B #4 buck that I bought patterns so badly in my shotugns, that it is pretty much unusable. It gave the term "scattergun" a whole new meaning.
My whole point is with regard to shotguns in particular, there are no pat answers. Pattern your gun with several different loads. Practice. Learn to quickly re-load, a particularly important skill with a single shot. Find out what will work for you in your circumstances.