Most people are going to shoot it at a piece of paper.
Eric_A asked if 16 inch was louder. 16 inch is noticeably louder than 20. I don't suppose that matters, though maybe he wants louder to impress his girl friend.
16" is also lighter, does that matter?
You don't need as long a case for a 16" and it doesn't take up as much room in the safe.
If you get into combat inside a building, the 16" may give you and edge in target acquisition. Remember though, it will be loud, so wear your ear protectors.