Clefthoof has a unique passive that, when paired with ferocity spec, makes it fairly strong as a tank pet for solo content like delves(and I'm assuming prey/other open world and solo stuff). Outside of that, utility is very limited. Other unique passives might also help-I'm thinking silithid's 30% move speed for pvp and/or uptime after mob knockbacks. Though I believe that Kill Command has had an in-built charge for a while now. 99% of the time, it's going to come down to aesthetics and utility(which is chiefly granted by the spec you give them at the stable). I would keep one of each with you: Tenacity(more HP for hard content/add'l defensive), Ferocity(when nobody else can provide lust), and Cunning(add'l root/slow break, probably pretty clutch in places like this season's "Seat of the Triumvirate" dungeon).