I don't think anybody who loves the AR (me) dislikes the DMR. The DMR is my ToD by a wide margin, and I don't play much Firefight.
But my AR and Magnum K/D and Kph are both about the same, I'm +413 with the AR, +365 with the Magnum. DMR, It's like +1230. Obviously it's my go-to weapon.
I prefer having a full-auto weapon as my short-range fall-back. And I love killing Magnum-users with it, because all of them seem to feel as though it's a wildly inferior weapon.
It's also fun closing the gap in a DMR fire, tossing a grenade, swapping to the AR and annihilating them in close-quarters while they spam the trigger and hope to get lucky.