I have been using a slug gun for way to many years, From Remington 870's & 1100's, Beretta's Extrema Slug gun, H&R sluggers, Savage 210, to a Mossberg 935 slug gun. All 12 gauge from 2 3/4 to 3 1/2 inch slugs. I have killed a ton of deer with them, using all kinds of different slugs, but mostly Remington copper solids and Lightfields slugs. The 12 gauge is a great deer and bear killer, recoil has always been heavy using slugs, especially the 3 1/2 inch lightfields in my Mossberg. My most accurate 12 gauge slug guns have been the Savage 210 followed by my Remington 1100. Now the past few years I have used a 20 gauge slug gun on deer just to try them out, and they have worked great on deer, have not shot a bear with one yet. Just bought a Savage 220, 20 gauge to try out this year on deer, the accuracy is great, better then any of the 12 gauge slug guns I have owned. At 100 yards 1 to 1 1/4 inch groups, that I am getting out of my new Savage 220, better then some guys I saw shoot rifles. From the reports I have had, the Savage 20 gauge has been a great deer killer out to almost 200 yards. The other nice thing about the Savage 220, it is just like a rifle, shoulders a lot better then a shotgun with a slug barrel. As for your question which one is better, I guess in my opinion they are both good, dead is dead, but the 12 gauge makes a bigger hole.