I've loaded Bismuth and Hevi-shot for about 12 years, both of which are superior to steel shot. Bismuth is not made anymore, but ITX and Nice Shot are very similar in density, around 9.6gr/10.0grs per cc. Hevi-shot comes in at 12.0grs per cc.
I load 1oz of #6 Hevi-shot for the 20ga and 1 1/8oz of #6 for the 12ga. These loads hammer ducks. When I expect geese, I load the same amounts but use #4 Hevi-shot.
#4 Hevi-shot will shoot through mallards in excess of 45-50yds and crush geese. #6 Hevi-shot will cleanly take mallards over 40yds.
I've no experience with ITX or NiceShot.
Ballistic Products and Bucks Run are good places to get the shot and wads, as well as loading data.
