Ahhh...The great caliber debate.

The 62 grain M855 is steel core and therefore built to penetrate.

Can carry more ammo :advantage 5.56
Quicker response time shot to shot :advantage 5.56

Better long range balistics :advantage .308
Better one bullet stopping power :advantage .308

At top end velocities the 5.56 fragments well and this created an odd/nasty wound chanel.

Probably the best compromise is to have a couple of both rifles!