I would like to find a balance where maces are better in some situations, and hammers in others.
There's a bunch of ways to manipulate stats.
Maces are usuallly one-handed clubs as opposed to huge polehammers, right? You could drive them apart with the recovery numbers. Long handles mean you get the first blow in over the smaller weapons, so it could have a very fast windup (first number) at the expense of recovery time as you struggle to maintain your balance wrenching that monster back into a ready position. Smaller weapons are the exact opposite, trading out the first strike for the chance to get hits in while the hammer brother is still wrestling with his implement.
Toady does have a point with the hammers having smaller contact areas (since the things were more like metal picks than forge hammers), but even just the current 10 versus 20 is a really big power discrepancy with how physics works, so keeping those two close to each other is the best option for now. You could potentially fudge with the velocity values so a given swing with either would do the same thing to a guy's ribcage while the mace would mush much more of the surrounding flesh in the process, but that's fiddly work.
Penetration depth also matters. Maces and clubs with a low penetration value would crack bones inside the body, but leave the overall arrangement of the tissues intact. Hammers, on the other hand, cave in skulls and chests like nothing and it makes sense that they'd send bone fragments straight through lungs and hearts.