5.56 and 223 are essentially the same thing, 5.56 being the NATO round has higher pressures and shouldn't be fired in a 223 rifle however 223 can be fired in the 5.56. AR15 to M16 is a different animal, as far as I know the M16 is the select fire/full auto version, you don't wanna get caught with one of those, what you should be looking at is the difference between 223 and 5.56 in the AR15, which is the civilian variation. Most would say get the AR in 5.56 which most are anyway, then you can shoot 223 ammo in it, if you get a 223 you are limited to 223 ammo.
5.56/.223 are almost the same cartridge, you can shoot both out of a 5.56 chambered gun. But only .223 out of a .223 chambered gun.
That being said most AR type rifles are chambered in 5.56,I've yet to come across one chambered for only .223.
An actual M16 is a select fire military weapon,so its pretty much off limits for civilians unless you go through all the red tape to own a class 3 firearm.
I believe the question you are trying to ask is do you prefer a AR-15 carbine,set up with a shorter barrel (16") with a collapsible stock or a AR-15 set up like a M16 rifle with the longer barrel and full length solid stock.
I'd rather have an AR set up like a carbine, the extra barrel length may milk some more FPS out but is really not worth it to me unless your going to go with a heavier bull barrel to get the accuracy with the increased FPS. Which kinda defeats one of the primary strengths of the AR platform to begin with which is being a lightweight gun In my opinion.
If I want a gun to shoot out far with I'll just get a .308 or heavier caliber.
Depends on your skill for effective range. I have hit man sized targets at 750 meters with an M16, but that is beyond what is considered the max effective range. In some states it is not legal for deer, in others it is- my personal opinion- too light for deer.
BTW, if you want to buy an M16, you need to be 21, live in a state that permits it, pay a $200 transfer tax, and have a heavy duty checkbook. A legal, transferrable M16 is about $22,000 right now. No, all those zeros were NOT a mistake.