I did some fairly extensive tests on this. While the differences are small enough, it seems that maces > hammers against unarmoured opponents and hammers > maces as can openers.
Materials are... complicated. Density is good (yay for silver!). Toughness is good (yay for steel, since adamantine is entirely too light). Stiffness is good (yay iron!). Steel seems to be best against unarmoured or lightly armoured opponents, silver against heavily armoured ones. Since copper and bronze provide a good compromise, you can make decent blunt weapons out of pretty much anything - the differences are usually quite subtle.
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My preferences:
Bronze maces are balanced weapons for a uniform military. Also, if you somehow get decent morningstars, you have the right skill for a very useful weapon: Flexible weapons have their benefit modeled (high velocity of the business end) but not their drawback (not the full weight of the weapon is behind the blow). Scourges get all the press, but morningstars are generally better (not quite as fast, but more substantial).
Silver hammers are another good choice, as they are the most reliable can openers. While they are also the weakest real weapons against unarmoured opponents, all blunt weapons suck compared to the alternatives there. Side benefit: bashing opponents with crossbows isn't very effective, but it does use the hammer skill.