Out to 300 yards for your use, it's preference. The 308 has more penetration which could be useful dependant on cicumstances, however, the 223 would generally serve you well with the plethora of good projectiles for it.
Personally, I chose the 223 for this role. If you were to go beyond that distance to any significant degree the 308 starts to make better sense.
My theory is three 223s for every 308. The 223s provide security for the DM with the 308 and give him time to work.
