I think he meant the steel would rust.
But yeah, the 70 series is stronger than the 60... if all you plan on doing in plinking and range use, 60 is fine.
For serious use, the 70 is better, especially if you plan on doing lots of hard training with the rifle. When I was in the Marines, we used the rifle to two man lift a 3rd person over walls, plus melee training for close quarters and running out of ammo... I don't know about the carbines, but the A2 was a tough rifle... Most people will never even get anywhere near that level intensity of use with the rifle.