I'd still take the increased aim of 5.56.

If you put enough shots into something, [i]it will die.[/i]

Increased damage but less accuracy = Theoretically gets the job done quicker, but not many shots will hit from less accuracy.
Increased accuracy but less damage = Will hit more.

Honestly, if you're going up close and personal with something, that's what a shotgun is for. Fill em up with buck shot, Assaults.

Shoot down the muton with more accuracy.

Blow the berserker away with increased damage.

This is where teamwork and squads come to play, guys. It isn't WE CAN ONLY CHOOSE ONE WEAPON FOR EACH OF US. It's a squad of different people. Some are going to take the 7.62 and some will take 5.56