The Definitive Dark Knight Guide

[Meythia]

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.

Unleash also does not break current weaponskill combos like Abyssal does. Besides, having the radius AOE around yourself can be quite more comfortable than around a target hitboxwise. (big enemies tend to eat the whole radius up even when small ones are directly next to them, See A2(S))