Hi Mike and Stephen
My bet would be for copper coating
if we are talking a straight choice between the two systems without complicating factors like corrosion, lubricants etc. I would only consider this though, if only one of the mating surfaces were coated (or bead blasted if we look at the other arguement). A bit like running a steel journal within a bronze bearing. Copper on steel will have much less tendency for forming mutual adhesion together than steel on steel. Copper will also act as a more accomodating soft bearing surface.