Personal preference really, one is proactive one is reactive. one mitigates damage the other heals it. I have a Necro/Pain and find it a bit easier than Dark pre 40 ( for healing at least ). The /Dark AoE heal is centered on you and a small radius, this means you need to be in melee with your pets to heal them. On the other hand after 38 you can use Fluffy to heal your pets.
I would have gone Necro/Dark but I already have a Mercs/Dark and wanted to have my 4 MM's all with different sets
I've enjoyred my lvl 24 necro/pain more than my 35 necro/dark (also had a lvl 35 necro/poison). It's more survivable so far, and definitely not as annoying. Healing with dark is really a PITA.
I want to make a necro MM, but can't decide which secondary would make him more survivable. The heals and resist buff in pain look nice for necro, but darkest night and dark fluffy look nice in dark.
Any opinions?