I hunt with both.
The 20 guage is plenty enough for most duck hunting senerios.
However, the 12 guage does offer a little more distance with a better pattern (2 3/4 in.).
But the 20 guage is more fun because it makes it more of a "challenge" to hit the ducks. It is a fun little cartrige that has less recoil (if that is an issue).
If you do go with the 12 guage, why only the 2 3/4 in., almoast all new and shotguns manufactured in the last 30 years are chambered in ATLEAST 3 in. So why wouldnt you just use the 3 in. if you were trying to gain extra power?
Hope you make a good choice for you.
[b][size=14]REAL Trucks don't got Spark Plugs
"Hold the Shotguns.... Its hard to stear like this!"