Both had identical effective range, although the M14 had a greater Max Range.
The M16 had greater muzzle velocity. But early variants due to the twists of 1:14 were a little unstable. The so called tumble does occur on impact, but the round does not continue to tumble inside the body. The turning over effect of a .223 or 5.56 round on impact was not something new to the M-16. In the Seventies Marines I knew who used both in Vietnam claimed the M-16 round could be deflected by a tree twig. I have read that may have been true of early 1:14 models and mismatched ammo.
The two biggest complaints about the M14 were weight and through and through hits on a small enemy that did not utilize body armor. It did not knock down the target. Not sure how fair those assessments are. It did weight a little less then 3 pounds more than an M16. Supposedly, you carried the same amount of weight because you carried more ammo for an M16. It took a few tweaks before the M16 functioned as an effective combat weapon. They were still tweaking ammo and twist during the Iraq war. With time, changes in ammo and polymers it may have been possible to reduce the weight of the M14 and develop a round that created more hydrostatic shock at close range. I think had the M14 been kept around and tweaked the way the M16 was, it would have been a better weapon. There was a lot of Army politics going on favoring the M16. It was always my impression the the Corps liked their M14s. They reluctantly gave them up in the Seventies. I know my DIs preferred the M14 over the Mattel model rifles.
My father is 95. At 19 he was packing an M1 pursuing fascists across Belgium, Germany and CZ. He served in Patches 7th and Patton’s 3rd Armies.
He still likes rifles. The last deer he shot, was a 400 yard kill. He did that with his 30-06. (same as M1) He has a caribou mounted with antlers in velvet on his wall in the den.
So, as far as arguing over .223 ammo vs .308 ammo, I think he has long understood that the 06 was superior to both of them.
Note, he at one point was an armorer and worked on Quad 50’s and Borors. Both Big Bore stuff. Think he still has a couple of .50 BMG bullets with link connecting them.
There are all sorts of "internet commandos" on YouTube telling us that the "M-14 is impossible to handle in full-auto" while firing the thing from the offhand (!). No offense to those bozos, but firing full auto with anything in combat must be done from the prone and preferably with a bipod. With a small amount of practice, the '14 could fire three round bursts quite surgically.
As for the weight and length of the M-14, I had this discussion with a high school buddy who served the 82nd Airborne over there - he said that the "M-14 was too heavy and its ammo too heavy for Vietnam - the M-16 was the right weight and we could carry more ammo".
I told him that "I wish you'd told me - I would have sent a big, strong Marine to hold the rifle up for you".
He's been pretty quiet since then.