BTW, the 12 Ga has quite the kick. But at 6'4" and 235, it's a draw ...
I have a 12-ga pump also.
The advantages of the pump include being cheaper and being less likely to jam when dirty or firing unusal loads. But you have to put in the time practicing with it to where operating the pump correctly becomes so automatic that you're going to do it right even under major stress (like combat)
The 20-ga semi-auto is better for guys smaller than me (I'm a pretty big guy, and it kick me pretty good), and for people without experience.