The other difference, for those of you who didn't notice by reading the link, between the DP and DPM was that the DPM was a bit more resistant to overheating since the recoil spring was moved from the gas piston right under the barrel, to behind the bolt carrier. Killing the temper on the recoil spring would SUUUUCK.
The DPM is the only choice for RO:O, really. Nothing else is readily "man-portable," and doesn't fit in with the squad-level role it would serve. It's the only one of the bunch whose purpose is even comparable to the machineguns already in-game.