As much as I like to "borrow" my father's old SXS 16 gauge, I would choose the 20 gauge for the following reasons: 3" shells throw about the same amount of shot as a a 2 3/4" 16, you can use 2 3/4"" shells in the 20 for practice or other small game with less recoil and typically lower cost, you may not always live near a store that has a ready supply of 16 gauge shells, unless you have other children or plan to keep the gun as a keepsake the resale on the 20 will be better, and lastly I believe the gun will weigh less. I have not seen a real world difference in the quality of patterns between a 16ga and a 3" 20ga. That said it won't make that much difference.
Probably didn't help much, did I? :grin:
Dave