Well, if your trying to keep the single Boss Occupied, then Heroic Slash is the way to go. If your trying to take down multiple mobs, which is really the DPS's job, then Deep Slash for the multiple enemies.
Either one gives nice added effects good for tanking, so it all comes down to what your tanking on. Me, I would prefer to load up single target abilities than AoE abilities for traditional tanking. Heroic can help you with Ultimate Gain, something that is harder now since we don't get Ultimate from Crits anymore like we used to.