Answer: Needle Rifle
1. You can carry more ammo for the Needle Rifle.
2. Needle Rifle carries more ammo per clip.
3. Bloom has more of an effect on the DMR than the Needle Rifle. I can fire the Needle Rifle as rapidly as possible and it hits who I'm aiming at 85-100% of the time. This is why in the old Team Classic playlist (the one that was out before Anniversary was released), the Needle Rifle was one of the starting-weapons along with the Assault Rifle; because the Needle Rifle, in Reach, is the gun most like the Battle Rifle.
However, the DMR's scope zooms in further.