Both factions have good reasons to go after Garrosh. It's really hard to say who's suffered more. Garrosh has done some major damage to Alliance, starting with Ashenvale and then the late bombing of Theramore. For the Horde he's causing mass of dissension amongst the people he's supposed to be leading and protecting. That's not the horde these factions originally joined in the past. Change is inevitable, yes, but to the degree Garrosh did was unnecessary. That coupled with turning on the trolls... *shrug*
Both sides seem to be neck and neck to me, and either way, both sides will be involved in killing him. It would be better if Blizz could figure out a way so that the killing blow came from a combined attacked from at least on person on either side, if not more.