Depends on the intended use of the rifle. 7.62 NATO for longer distance (or light anti-materiel duty), 5.56 NATO for closer distance work (or interior work).
Definitely handle (if not shoot) both models before purchasing. I have small hands and I cannot reach the mag release on the HK91 with my trigger finger without shifting my grip on the gun. This is purely an issue with the HK91 as I can reach the mag release on a AR10 with no problems. This becomes a problem when doing speed or tactical reloads. You don't want to buy a gun that doesn't fit you.
Justin
