I would recommend heavy-shot if you're going to be using a 20g. The extra price in shells would closely compare to shooting 12g. I have killed birds over decoys with 3" #2 steel on more than one occasion with my 20g, although I don't make it a habbit.
I think practice and patients is the hard thing. Wait till they are without a doubt in killing range for the weapon you are using. I know that kind of goes without saying but I know I'm guilty of shooting too soon. Just a thought.