Brian,
I happen to own an amazing set of books on metalurgy that I rarely look at. Thanks for suggesting the research on this. It will be a great excuse to get some use of those books, once I am home from vacation next week.
I think I am a bit confused on the whole idea of one type of bronze wearing the crank more than another. After all, the point is for the crank to ride on film of oil that is 0.0005"-0.001" thick, not to be in contact with the bronze. Perhaps the concern is for which material causes the worst damage if the bush is starved of oil. Or, perhaps, the concern is for when the oil carries abrasive particles. Then, the particles will grind away at the crank as well as the bush and it would seem the harder bush material would be better.
I will be interested to hear the ideas and, perhaps, warnings and admonishments of the many here with deep experience (in motorcycles, I already have expereince in the other).
Richard