I posted this on the turkey hunting site but nobody seems to want to answer my question. I am going to buy my son an 870 youth model. I noticed Remington makes the 870 youth model in a 16 guage with 2 3/4 only. Would this be better than a 20 guage 3". Which would have more range? And how much selection is there in 16 guage ammo?

I'd go with the 20 ga.....and for turkey get a box of Estate 3" 6s

For dove hunting later the 20 will shoot for $2.99 a box and the 16 will be twice that. The 20 is an extremely versatile Gauge and it's all I use anymore except for some 28 Ga hunts.

I also would have to vote for the 20 guage. The selection of ammunition available is far greater than the 16.

I agree with the other guys. Even though the 20 is really an inferior gauge based on shot collum length and bore diameter it is still pretty effective. And the ammo is cheaper and easier to find. The 16 is a great upland gauge but ammo is sometimes not readily available except at a premium price.
You never said how big you son is? If he is big enough, why not buy a full sized 870 express 12 ga. and cut down the stock if you need to (or have a gunsmith do it). He can shoot light loads with it and still be able to shoot 3" turkey and steel waterfowl loads come season. He won't feel the recoil shooting at game.
You can alway buy a replacement composite stock later on when he grows a little bigger.

The 16 guage is a WONDERFUL thing but gettin' ammo can be a PIA. I'd stick with the 20.

Put another check in the 20 gauge column. While I wish the 16 gauge was more popular and had better ammo support, because I think it'd make for a near perfect upland gun, it just isn't so.

When it comes to shell selection, the 20 gauge is second only to the 12 gauge. 20 gauge shells are factory loaded for every application a shotgun is suitable for, and the 20 gauge will kill anything a 12ga. will as long as you understand it's limitations.

The 870 Youth gun is a great choice, too. I learned to shoot shotgun with my dad's old (full-size) 870 Wingmaster 20ga, starting when I was about 6 years old. I was a taller/bigger than average kid, but it was still a bit of a handful size-wise until I was 8 or so. Every time I think of that gun it brings back some very fond memories...but I digress.

Just a suggestion, but also consider the Browning BPS Youth. Since I bought my BPS I can honestly say that it's the only pump shotgun I'll ever own. The thumb-tang safety is great (never liked cross-bolt safeties myself), but the bottom eject isn't for everyone (makes loading one at a time at the trap range a bit of a chore).

Good luck.

Mike