RE: 20ga vs 12ga for slug gun
I have taken deer with both gauges and have had no problems. I personally perfer the 12 gauge most of the time. I like that little bit of extra energy on the other end. I have a newer 870 with rifled barrel and scope and an older 1100 with an smooth bore slug barrel and love them both.
Go the the local store where you okan on buying and look at several different models in each gauge and find which one feels right to you. It is like gettign a bow, it comes down to the features you need and can afford and what feels the best when you pull it up to your shoulder!!!!!