As biased as I am towards the 16 gauge, I would suggest you go with the 20 gauge youth model for a couple of reasons: 1.) youth model shotguns are generally designed to fit someone who is smaller framed (the stocks are shorter and the forearms are moved back a bit). 2.) if you have trouble getting ammo, you might be better off going with the 20 (although I feel the 3" 20 gauge 'magnum' is an abomination, just like the 3 1/2" 12).
FWIW, there are several 16 gauge nontox loads on the market, including steel, bismuth, and tungsten matrix. In the catalogs I order from, non are more expensive than comparable 12 gauge loads, so if you are paying through the nose for 16 gauge shells, you are getting ripped off by your local dealer.