Unleash's use is being slightly cheaper MP-wise.
Abyssal Drain costs about 100 more MP at level 60 in exchange for being usable at range and dealing slightly more damage (120 potency versus Unleash's 100), and giving an HP drain effect when used with Dark Arts.
If you're not going to use Dark Arts on Abyssal Drain, it comes down to a matter of taste.

Also I personally just don't like the animation for Abyssal Drain.