I don't think and M15s were actually issued, anything that looks like an M14 with a bipod, pistol grip, and foregrip is almost certainly an M14A1.
pictureIt's also possible that you'll see a few M14 (modified) that don't have the pistol grip and the foregrip, but instead just have the bipod and heavier barrel.
Since the Marines were the first ones to be issued the M16 rifle, they had the most problems with it. (See stories of Marines duct taping the M16 cleaning rod under the M16 to be used to 'ram out' jammed shell casings, and other M16 stupidity). If the choice was between the 1903 Springfield and the M16, the Marines would have chosen the Springfield.