The AUG A3 for sure.
Very much is like Colt vs. Bushmaster. Both are fine rifles but the AUG is much better made and costs more. The AUG also seems to be more accurate, but I haven't determined if it is the rifle or the MSAR optics that are the cause. Simply put I'm too lazy to do a side by side of both rifles with the same scope. That and the MSAR shoots well enough for me not to worry about it too much. By the way you can run AR mags on a factory Steyr with a NATO stock. But if you want caliber variation, MSAR is your only option.