It's been stated that the Directed Marksman Rifle(DMR) and the Battle Rifle(BR) do the same amount of damage, although the BR spreads that amount of damage over the course of three bullets. It is true that the DMR has a faster rate of fire, but it is untrue that the DMR does the same amount of damage to anywhere on the body, that a BR does to the head.
A single shot from the DMR is = to one shot from a BR, spread over three bullets. It IS possible to drag the bullets from a BR shot into a spread, therefore increasing the area that can be hit. This however, could cause the enemy to only take one or two of the bullets from the BR. If you pit two inaccurate players against each other, jumping and moving around, one with the DMR and one with the BR, the BR will win because of the higher chance to hit (Theoretically of course. There are factors to consider, including terrain, power-ups, and grenades.).
If you're a better shot, USE THE DMR, IT IS BETTER IN THAT SITUATION.
tl;dr use a DMR if you're good; BR if you're bad