There's 3 concerns here:
First, since it's a mace (blunt weapon), the material's weight determines how much damage it will deal on a hit. Now, silver is heavier than brass by about 1/4, thus on a hit it will deal slightly more damage. However, all indications are that the difference is quite slight, so we need to look at the other issues.
Second, quality. It looks like item quality on blunt weapons does nothing beyond affecting accuracy (since they have no edge anyway). An artifact weapon multiplies the user's skill by 3, then adds 5, which naturally affects their accuracy. Your "high quality" weapon, I'm guessing is Exceptional (which multiplies by 1.8 then adds 4) or Masterwork (x2+5). So, the artifact is significantly better, except that there's one more thing we need to look at.
That last thing is the intended wielder's skill. Most skills' effectiveness stops improving after level 20, and your Mace Lord has at least 11 skill with maces. I'm uncertain as to whether weapon skills are among the skills that continue to have an effect after 20, though I'd guess that they probably do, but there comes a point where you're hitting 100% of the time anyway so the improved accuracy doesn't actually matter. With an Exceptional weapon, a Great-skill Mace Lord will have an effective skill of ~22, so if the effectiveness of skill is capped at 20, that's as good as any weapon.
To answer your question directly, I'd say that the silver mace is probably very slightly better unless you're going up against very, very big things (which are harder to get a solid blow on).