I'm in the plum position of both liking Orcs best for their model/animations, fantasy/lore shit, and broken-ass Hardiness.
That said I'm rolling an Undead Warrior as my primary alt, which is maybe the worst Horde race for Warrior, just because I'm taking him less seriously as a warrior. So in that case, I guess for Aesthetics and for doing a different start zone for my second character.
Only real thing I don't take into consideration is PvE performance, I guess. I think everyone can get the job done there pretty well (don't have to worry about Fear Ward on Horde side), and raiding's not really on the top of my priorities.