I-6's point, supported by a number of others, about the need for a section/platoon/company "armoury" from which the right weapons can be selected seems to me to be about right.
If you are doing most of your fighting lying down (prone position, stable platform, engaging targets either singular or in hordes as was the case for NATO and is the case, if I understand correctly, for FOBs), tactilcally defensive, then 7.62 is the answer. The logistics of the heavier round are less onerous when you have gone firm and the supply system can keep you fed.
If you are doing most of your fighting on your feet (patrolling, advance to contact, meeting engagements, only supplied with what you can carry, engaging targets around the next corner and up the stairs) then a lighter round, 5.56 makes more sense.
If you can't get one rifle/mg combination that will fire both calibres, and from what I can gather on this and other threads that is not completely out of the question with this talk of interchangeable uppers and receivers, and M16s being fitted with Gas Pistons to make them operate like an FNC1 or an AK47, then at least you can get the same weapon in two different calibres and keep them both in stores.
At least you will have common training, drill and handling. And the field commander gets to decide if he wants a room clearing or rapid reaction assault section in his defensively oriented platoon/company, or if he wants a heavy support section in his platoon that has been tasked for a clearing mission.