I would pick the 20 if you were going to be using it only for deer hunting, if you were looking to use the same shotgun to hunt lots of different types of game i would go with the 12. I used to hunt with an 870 12 gauge before the area of NY i hunt switched to rifle. Used to shoot Remington 3 inch copper solids and they were butt kickers on both ends. 20 gauge will be a lot easier on your shoulder and the deer will never know the difference.