With all the improvments we've witnessed in the last few years,ballistically,they're about equal as long as you are comparing sabot slugs. If comparing the older foster type slugs,the 12 gauge wins. In THEORY ONLY,the 20 gauge should be a tad more accurate due to the thinner sabots,but if it is,I havn't witnessed it. Go forth and slay your targets knowing you'll be able to stay with almost any slug gun.