Incorrect data as of version 4.01 patch
As of Stormblood patch 4.0 there is a major difference in enmity generated from Abyssal Drain and Unleash.
According to this reddit post Unleash generates 515 enmity when you use it and Abyssal Drain generates 600 enmity per use. So Abyssal Drain trumps Unleash when generating enmity, so that brings up the question.
When should I use Unleash over Abyssal Drain then?
One major factor to take into consideration when deciding what enmity booster to use for AOEs you need to pay attention to how the skill works. Unleash is a slightly weaker AOE and generates less enmity than Abyssal Drain, however the skill is targeted around you and requires no target where as Abyssal Drain requires a target and is based around the target, not you. This trait of
Unleash is helpful for starting off group battles or when the enemies are surrounding you. Abyssal Drain targets an enemy and performs the AOE around the enemy so if you are surrounded by enemies Abyssal Drain may miss enemies around you. Once you get the enemies you're fighting grouped up properly Abyssal Drain is your best bet for generating enmity.
Another thing to take note of is the MP costs of the skills, Abyssal Drain costs more MP to use than Unleash does, so if you are hurting for MP at the time it may be beneficial to use Unleash over Abyssal Drain and take the small hit to damage and enmity generation.
Patch 4.01 updated answer
As of the 4.01 Omega patch the modifiers for Unleash and Abyssal Drain have been modified again. In this patch Unleash now has a 20.6x modifier giving a enmity potency of 1030 whereas Abyssal Drain remains with a 5x potency giving 600 enmity. With this update Unleash is the more powerful enmity generating AOE attack for the Dark Knight. It is still important to remember how the skill is used and Abyssal Drain still does more damage but Unleash is now the most potent skill for Dark Knights to use to draw aggro from enemies.