The M14 has significant advantages over the M1. I trained on the M1 and carried one during my first tour in Viet Nam (after my issued M2 carbine got wrapped around a tree.) On my second tour, I bullied my battalion commander into getting my company two accurized M14s (pre-M21 sniper rifles) and kept one for myself. So I have used both in combat.
1. The M1 was a high-risk item. It was difficult to manufacture -- so much that during WWII only Winchester and Springfield Armory could produce M1s meeting Army standards. Much of this was due to the overly complicated en block clip mechanism.
2. The M14 gas system is much improved over the M1's direct impingement system. The M14 uses a short-stroke hollow piston. The piston is open at the front, like a water glass, and gas is vented into the interior of the piston. As it moves back, the hole in the piston moves out of alignment with the hole in the gas cylinder. This makes for a much smoother gas action -- in essense, the M14 is driven by expanding, not by impinging gas.
3. The gas piston design makes the system self-adjusting. Too high a pressure, and the piston moves fast, cutting off the flow. Too low pressure, and the piston moves more slowly, allowing more gas to enter. That's why the M14, unlike the FAL, doesn't need an adjustable gas regulator.
4. The short-stroke piston strikes a counter-weight, which smooths out the action even more. The counter-weight operates the action as inertia carries it back.
5. Unlike the M1 with its camming surfaces, which had to be greased, the M14 uses roller bearings at critical points.
6. The 7.62X52 NATO round achieves the same ballistics as the M2 Ball .30-06, but with a case 1/4" shorter. When you are manufacturing billions of rounds, the savings in brass are enormous.
7. While the M14 was capable of full-auto fire, this was accomplished by the installation of a selector switch. Selector switches were issued separately. Wise company commanders locked them in the company safe -- and forgot the combination!