In tier one the Ammo mod is more efficient. It accounts for a 33% increase to ammo, vs a 28% increase to damage. If you're strictly looking at damage per resupply take the Ammo.
It might be different in terms of breakpoints on lower Haz levels, but on Haz 5 with T1 ammo: 3 shots to the grunts mouth is a kill. 2 shots if you take the electric dot or have full locks.
