I'm looking to get some bulk ammo for my forthcoming mp15 and have some questions....

1) what does the difference in gr mean to me?
2) what constitutes 'good' ammo. I see some ammo listed as "great for plinking?" I assume that is code for low quality?

3) what should i look for in ammo that may be stored for some time? What's the best way to store it?

What be the difference in THIS AMMO @ 500 for $130 and This ammo @ 500 for $200 or This Ammo @ 500 for $155.

Much Thanks!