Some penetrator lying around is OK if you're going to shoot through car windows/doors and such training or for defense. Slightly less effective at penetration but much more effective on soft targets is the Federal 5.56mm MK318 MOD 0.
A friend was in on the devlopment of the round and he uses it deer hunting....I don't know why not a hunting round but whatever because it is cheaper than premium hunting rounds and works for him. What the MK318 lacks a bit in penetration it makes up for in terminal performance and..it's claim to fame is not being deflected so much through, say a car window, when you need it. So if you're stopped inside your car and need your AR to shoot through the window or vice versa to stay alive you'll want that MK318 ammo over green tip. It's what I see Tier 1 operators using when they train at the range...they have stopped carrying green tip. Take that for whatever truth you care for it to carry.
If I had no penetrator at all I'd get some amount of green tip just to be able to shoot back through concealment more effectively and because bad guys are increasingly using body armor when they kick down doors for home invasions.
My go bag has no green tip but it does have some MK318 in it. My home defense AR has MK318 first round or two. It's expensive if you can find it ($15 for a 20 rd box), but you should have a box or two in preference to green tip if you perceive a need.
http://palmettostatearmory.com/index.php/ammunition/rifle-ammunition/223-5-56/federal-5-56mm-mk318-mod-0.htmlNatchez Shooters Supply is almost always the lowest cost 5.56 provider of green tip or fmj.
www.natchezss.com