If it's a gun you really like and see yourself using a bit over the next few years why not buy at a box each of at least 2 brands and shoot some paper to see how they pattern. For me I tend to go with shells that are a better value. Bismuth shells are not cheap but at least they perform decent. All that said I've never used any so I can't give you advice on any. But you can also look at Kent Matrix (very expensive) and Hevi Shot classic doubles. Both can be shot through older shotguns but are not bismuth. Enjoy shooting the old gun!
Dear Lord please bullwhip me for saying this but I agree, Marcstar is in the lead. Please nobody use this as a quote!!!