Not sure what the difference is between a 12 gauge shotgun and 20 gauge shotgun. This video is a great demonstration showing the difference between the two.
I started hunting when I was 12 years old and I started out using the 20 gauge shotgun first. I loved using the 20 gauge as I was afraid of the kick when shooting the 12 gauge. I used the 20 gauge for about 2 to 3 years. But now that I am older and realize you don't feel the kick when shooting I have my very own 12 gauge which I love. Don't get my wrong I loved using the 20 gauge but it didn't have the power like the 12 gauge does.
For first time beginners, using the 20 gauge wouldn't be a bad idea. I know when my children start hunting I will have them using the 20 gauge first just cause I don't want them using a gun that is to over powering for them. But again it is up to you on which gauge you would like to use for hunting.
I feel using a 12 gauge for waterfowl hunting is better than a 20 gauge as the 12 gauge has more power and can reach a little farther especially if you are going to be hunting on land. We all know everyone does a little bit of sky busting when hunting on land.
Everyone has their favorites and there is nothing wrong with that.