From my experience with a 1984 johnson 70hp, definately stainless. It chews up and spits out rocks, half submerged drywall buckets and wood. The prop has a rubber bushing what will allow the prop to spin if too much pressure is applied. Now I don't hit any thing at full throttle..often. Hit four of those drywall buckets at full throttle in one day on the Ohio in West Virginia. Guess they were building a new apartment complex up in the hills some were. I used to have my props rebuilt 2-3 times a year while fishing the falls of the ohio and at $75 a pop that makes stainless is much cheaper.

It became real clear how delicate aluminum props are when I dropped one of the steel clips from a ski rope on it while idling and it bent it as I watched. The clip only weighed about 4 ounces if that. It is not a question any longer for me. Boat also preformed better with steel...I guess it is more rigid.