Well, I do know a little about the relative prevalence of maces vs. hammers, and it seems that throughout the age of plate armor (1300's till 1600's), single handed maces, hammers, and axes were all used frequently by knightly warriors on horseback. On foot, whether peasant, noble, mercenary, or man-at-arms, pole weapons were used with two hands seldom with shield, whether with an axe-like (halberd), spear, hammer, or mace head. As hammers and maces were both used in the same periods with similar frequency, I would say that IRL each were certainly effective, probably equally so, though I can't say for sure since my source for this information (Arms and Armor of the Medieval Knight) doesn't go much into matters of design considerations of blunt weapons other than that they were meant to be used more against armored opponents.
Hmmm... perhaps durability might be an advantage of maces over hammers, but some maces have similar construction to warhammers with wooden shafts. Some maces have metal shafts.... I don't know how much I buy the backhand hypothesis posited earlier, since I'm having trouble imagining a situation where this would be the case, but it could be true.
One thing that maybe could be an advantage/disadvantage (though I don't know if it's realistic) is maces to not being as able to be stuck within the opponent as easily as hammer heads maybe perhaps maybe. I'm going to have to go hunt for more sources on the matter.