Either will kill you dead. The .5.56/.223 may have better range, but how many people can hit something at 500 yrds?
As far as reliability, I never had any issues with the over-worked, wornout Air Force range M-16s. My son's been shooting Mil firearms (M-16, M4, Berettas, etc.) for fifteeen years, says he can count the total number of failures on one hand and have fingers left over, and doesn't understand why so many seem to have issues with firearms feeding/firing.