First, welcome to the forum.
The harder cup primers like the CCI 450 or 41 are better as is the Remington 7 1/2. The reason for this is that the free floating firing pin in the AR rifles will hit the primer when the bolt slams home. There is a fear that this will cause the round to fire which is the slam fire you will hear about. I don't know that anyone has ever documented this happening but there is always a possibility. The possibility also exists to a lesser degree with the harder primers.