On one hand, the 16 would put more shot downrange, but ammo is much harder to find. I think it would be good to teach them that they need to make the first shot count, and the 16 single shot would do just that. But, on the other hand, the 20 ga ammo is readily available and in more loads. The 20 is a magnum, so the shot # differential is probably negated. You can teach the first shot thing. The recoil between a 16 and a magnum 20 is about the same. You might also want follow-up shots in deer, duck, rabbit, squirrel, or dove hunting. So, all things considered, I think I'd go with the 20.