I'd go for Ret/Rogue without a doubt. So much CC and so much survivability. DK, after they're nerfed of course, I reckon will be a bit squishier and they can't really run away as much as a rogue can.
Easiest to make work right now will be Ret/DK. Like literally its easily a top 5 comps in 2s because of the way healing works and DK survivability. Both classes are very hard to take down even by other 2 dps comps and you have a shot at killing healers. Dk are likely to be nerfed thought, so its really uncertain. Ret/rogue is about as strong, but its not fool proof, the rogue actually needs to know a little bit about his class and you guys needs some coordination even if hes mutilate instead of sub, while DK/RET is pretty much just about sher tankiness and damage.
If you can find a good rogue, id go ret/rogue. If you have only choices of "ok" players the DK will certainly be better.