Eliminate platform (assume Ruger Mini-14 or Mini-30, for example) and price (both with readily-available surplus practice ammo) from the question. Please explain your choice.
All the after action (admittedly anecdotal as your own observations) reports from the sandbox indicate some pretty wide stopping power dissatisfaction with current M855 62gr ball in 14.5" M4s.....amusing assumptions on the 5.56, I used it for many years, in hot zones...and it seamed to work just fine.
Not trying to sidetrack the issue, but where did you get this from? I would use a good SP or HP round, as would most here, I think.In ball I would pick 7.62x39mm. And I say ball because that is what most of us plan on using as defense rounds. So that eliminates that ammo selection argument.