The polys would be more durable in the long run, if the feed lips on the metal mags get hit they could bend and give feeding problems. The poly mags have "give" to them and it would be real hard to tweak the lips. The metal mags look nice, more of the M-14 look, but rounds have to be slid front to back. The poly mags can be top loaded, in or out of the rifle. Nice to top off a mag without having to remove it. That's provided you have a scope mounted in the "Scout" position out of the way of the action. You do have a scope mounted out over the barrel on the GSR, don't you ? There are about 6 reasons I can think of why you should put the scope there and not in the conventional spot over the action. Lastly the poly mags are cheaper, lighter and are a bit shorter than their metal counterparts.