When I was bying for my son two years ago (he was 12), I looked at most of the same. I have a Benelli Nova combo, but the youth version is not 'upgradable' for size because of the one piece stock. My choice came to either the 870 or the Mossberg. While I like the safety on the Mossberg, and it is a fine gun, I went with the 870 in 20ga, well...because it's the 870. I got the combo. He has killed two deer and many pheasant. I'll be changing to a full size stock this year.

As a confession...what I did was pick up a second used 870 off of Gunbroker for a great price and will be getting a Boyd's thumbhole stock for the deer gun and have a second complete gun for pheasant season, as well as a short (youth) barrel and longer bird barrel. Can't have too many guns.