for my horde toons, I've gone with what I think best reflects their class. so Pally, Priest, Monk, Warrior are going Kyrian; DK, DH and Mage are going Necrolord; Hunter, Shaman and Druid are Night Fae and Warlock, Rogue are Venthyr.
then my alliance toons/other horde toons will fill any xmog gaps; so Hunter in Kyrian, Shaman in Necrolord and Venthyr, DK in Venthyr as well and Paladin/Mage in Night Fae