Except that was Grommash that did most of the work, regarding Mannoroth and the Legion.
And Garrosh did NOT control an Old God's Heart at all. This fucker knew what he was doing.
- - - Updated - - -
Both have shitty fucking cores tbh. Varian's core is imo much greater than theirs. Hell, even Arthas had a pretty strong core. The dude fucked over Ner'zhul and Zovaal at the same time, and ripped his own Heart out so he couldn't feel pain or emotion anymore. The badass.