Although I am out of the slug gun game now (rifles were legalized 3 years ago in my area), I shot slugs for 20 years. If I had to shoot them again, I would go with a 20 gauge. The ballistics of today's sabots in a 20 gauge rivals that of my muzzleloader, and I have shot enough deer with that to be impressed. The recoil of slugs is awful, but in a autoloader the 20 should be around .308 level, much nicer than a 12.
Will