I am a 20 gauge fan.
Maybe I'm too sensitive to the blast, but I don't like how loud the 12 gauge is when fired without hearing protection while hunting.
The 12 gauge slugs also produce more felt recoil if not wearing a jacket to help pad the shoulder when shooting.slugs at the range.
When I hunted with a 20 ga, smooth bore, I used the Brenneke 20 ga. 2 3/4" magnum slugs without any problems and they produced good results on deer.
When I went to a 20 ga. rifled barrel, the Winchester BRI sabot slugs also performed admirably.
Here's photos of a 20 ga. BRI slug that traveled diagonally from a quartering shot through a large deer and was found on the far side under the hide.
The slug expanded from the original .40 caliber to about .65 caliber.
While the 20 ga. may be preferable to me, that doesn't mean that it's better for everyone.
I can imagine situations where a 12 ga. slug gun would offer some advantages.
But its disadvantages are the reasons why I'll always stick with a 20 ga. slug gun.