I really like the idea of /Pain for zombies. World of Pain will stack with the resists zombies already possess and the heal radius and animation makes /Pain much better for keeping pets alive when compared to /Dark.
Necro/Dark has awesome debuff potential with the Lich+Servant+the MMs debuffs, while Necro/Pain is a very well-rounded MM with the Lich controlling and debuffing while you are healing/buffing/debuffing.
You can't go wrong either way. I would go Necro/Pain just because Necro/Dark has been done to death since day one of CoV 
Level 50s: to many to remember at this point
Its super easy to come up with a concept with Necro/Dark, and they do have some synergy. 2 fear cones, 3 holds, 2 AoE debuffs, 2 AoE heals.(counting Dark Servent, and the Lich.) It's a very safe combination.
I've never tried /Pain, but from what others have said, it must be beastly!
Can't go wrong either way, as Bry said. 
I'm looking to create my second MM. I've got a 50 bots/traps MM that was a lot of fun to play, so I'm familiar with basic MM playstyle.
I'm looking for anecdotal experiences, especially the "oh it would be nice if" stuff.
Obviously, necro is more melee oriented and that will bring with it a lot more micro of the pets (Bots/ rarely need to be told where to stand mid fight). As such, I'm torn between the /Dark and /Pain.
Dark appears to offer less to do during a fight which in turn offers more time to micro the pets. Lay down Tar Patch, anchor Darkest Night, and Twilight Grasp/Fearsome Stare/Petrifying Gaze as needed until done. I've watched Dark defenders and other MMs in action enough to know what to expect there.
Pain is the big unknown to me. I haven't played a lot red side since it came out and I just haven't seen it in action. I've tried playing a /Thermal controller and really didn't like the mid-battle healing. So my question is, how much healing would I be expected to do on a team? Obviously solo play would be spent healing the pets, but if I have to heal 6 pets plus up to 7 other players... that's a lot of targets to potentially have to apply healing towards. Still, Pain is new and new can be an interesting motivator for continued play.
How do the debuffs compare in practice? Does anyone wish they had Tar Patch instead of Anguishing Cry or vice versa? What about World of Pain vs. Darkest Night? I don't need someone to explain the numbers. I have Mids.