12 gauge, no doubt about it. The 20 gauge definately has its place in the world, but "most versatile" it is not. There are hundreds of different factory ammo options available suitable for every game animal from doves to moose.

If you don't mind a touch more weight and in some cases a bit more recoil (people seem to think that 12 gauge field loads are shoulder killers compared to their 20 gauge counterparts, but they aren't), the 12 is definately the way to go.

Mike