45-70, hands down. Nothing wrong with the 12ga slugs, and for what it's worth, some loads are almost identical in either (or some ARE identical), but I guess I think it simply makes more sense to shoot a rifle, rather than shoot a shotgun pretending to be a rifle. Accuracy with the rifle will almost always be slightly better, and the 45-70 rifle will likely be more wieldy in the field than a shotgun.