I'll give you point that you should Know about, and you'll decide.
- Cofagrigus has Better Defense by 20, while having lower Special Defense by 30. However, Cofagrigus can boost it's special Defense and his Special attack with Calm Mind, a thing Dusknoir can do too, but Cofagrigus can utilize the Spacial Attack Boosts, since he is a Special Attacker, unlike Dusknoir. Another thing to point out, Cofagrigus has better HP by 13, although not that important, it can actually help a little.
- Dusknoir is very good Defensively, and surprisingly offensive pokemon, although Physical.as far as defenses can go, base 135 in both is nothing to underate, because I don't think there is a Pokemon that can OHKO it with no stat boosts. Dusknoir also has the elemental Punches, and a pretty reliable STAB Physical attack in the form of Shadow Punch, and good Coverage moves like Brick Breakand can Cripple certain Pokemon with Will-O-Wisp and toxic, he is actually really good at Trick Room abusing, and can pull off some Massive damage With Pain Split, due to it's low HP.
Honestly, I would Choose Cofagrigus, only because he can utilize Calm Mind like hell, and can Damage and cripple the foe with Toxic and Will-O-Wisp.