To elaborate on the mathies here:

I shall measure MP costs in the context of "Siphons", this being the amount of MP restored upon one use of Siphon Strike. This is also the amount of MP Carve and Spit restores when used without Dark Arts.

Dark Passenger costs 1 Siphon
Dark Arts costs 2 Siphons

If we wish to speak of efficency in terms of most potency gained per spent Siphon, we can rank various abilities as such.

Applying Dark Arts to Carve and Spit: Spend 2 Siphons to gain 350 Potency. A gain of 175 Pot/Siphon.
Using Dark Passenger on its own: Spend 1 Siphon to deal 150 Potency. A gain of 150 Pot/Siphon.
Applying Dark Arts to Dark Passenger: Spend 3 Siphons total to deal 250 Potency. A gain of 83.33 Pot/Siphon.
Applying Dark Arts to Souleater: Spend 2 Siphons to gain 140 Potency. A gain of 70 Pot/Siphon.

Against a single target, Carve and Spit is your most efficient use of MP for damage. This is followed by a naked Dark Passenger and then finally a Dark Arted Soul Eater.