This is a "half gut" / "half objective" opinion - but I'd say it will make no practical difference in the long run, relative to the life or performance of your gun, whether you shoot .223 or 5.56.

On average muzzle velocities out of a 5.56 seem to be 200-400 fps faster. The gun is obviously designed to take max pressure, but available ammo won't be loaded that hot.

If that added velocity suits your application better that might be a reason to lean toward 5.56. Other than that - I'd say use the one that meets your cost and other objectives (bullet type, etc.)

I'm assuming, of course, we're not talking use in extreme conditions - such as high amounts of dirt, extreme high volume of rounds, etc.

OR