Ductile is significantly stronger than regular cast iron, and I'd be willing to bet that cast stainless is stronger than ductile for this use case. The information I've found states that cast SS is less brittle and generally more impact resistant than ductile. Maybe the cast stainless it easier to produce than ductile? I know that SS can be investment cast which typically results in closer to near net shape and doesn't require positive draft angles. So there are probably several good reasons why you would pick the stronger material.
