I started with a sixteen 870 back in the day and have always been a fan. I currently have 3 and unless one is after geese and or turkeys I have never found it lacking . Actually a high brass load of 6s would probably do a turkey in quite well. As to ammo I have never had a problem getting ammo despite popular internet legend mostly perpetuated by people that don't own 16s. Having said all of that as Shrapnel stated early on in this thread an oz of shot from a 20 ga works pretty much as well as a oz of shot is pretty much an oz of shot in the field regardless of bore size. All the babble about square loads etc is pretty much in the same realm as the ammo issue mostly an Internet discussion point.
