20s are quick...and don't kick as hard (and Encore's are light...they bite worse than that 150gr of Pyrodex does...and thats not exactly a .22)..but you do not have nearly the selection of ammo to choose from with the 20ga.

I would go with the 12ga. Its the standard...can't go wrong. As long as you aren't averse to recoil (cause for my money...it doesn't get much worse) then the 12 is all you will ever need.