The 16 is still the most popular gauge in Europe, but the 12 has always been number 1 in the U.S. and Great Britain. However, the 20 in the U.S. has a much greater selecion of different kinds of ammo than the 16, plus the 20-3" round which is equal to a standard 12 in a much lighter and handier gun.

I personally prefer the 3" 20 ga. for everything.