Most of Dusknoir's offensive power is from hitting super effective with the elemental punches, Earthquake and Focus Punch/Brick Break anyway.
I'd always go with Shadow Sneak if you're using a STAB attack on him at all. You've got a chance to take down weakened Gengar, Alakazam, etc. before they throw a Shadow Ball at him. It 2HKOs both of them quite easily. Also stops Reversal Lucario, who doesn't care if Sand Stream is active or not.