Kevin: Have used both #1 and #2 Bismuth, for geese. Both worked very well. Have no preference, between the two. Both hit hard. This, for pass shooting, as well as over decoy sets. For geese overall: #2 would be the most versatile, in my view. Have not tried BB...as the results I was already getting were good. May try some in future, though. To satisfy my curiosity, if nothing else.
I have also used #4, #5 and #6...but, this was exclusively on ducks. For in close, decoyed open water lake duck, ie: bluebill, bufflehead, etc., I found that either #5 or #6 were the ticket. Likewise: teal and wood duck. For pond shooting mallard: #4 or #5. With #4 preferred. Again, most versatile.
I also took a nice ,26-lb Eastern wild turkey with Bismuth. Using #4. My first one, with Bismuth. That was some years back. Have taken a few more since then. But, that one I remember best.
As with steel, the key to Bismuth is to get that velocity up a bit. Doesn't have to be a lot. Since Bismuth is only about 8%-10% less dense than lead. As long as you can duplicate lead shot performance, that is usually enough.
Al