Current thoughts amongst our safety helms seems to favour stainless props — we're trying to find one to replace the aluminium on a Honda BF50.
We've smashed a couple of aluminium props beyond repair, and had a few big-ish strikes with stainless props on the Mercurys, but never had any problems with either gearboxes or prop shafts. I've got a feeling we may have broken a couple of rubber sleeves on the stainless ones.
I once backed off the beach into a granite boulder lurking beneath the surface with a more power than usual, and that wasn't enough to damage anything beyond a bit of a dent in the trailing edge of one blade. Even the rubber sleeve was unaffected. I don't intend to produce a larger sample of experimental results, though