Tier. Tier helm has Hit/Haste, while the Dark Animus helm has Crit/Haste. You can also take the Retribution Tier Chest (Haste/Exp + 3 sockets for a whooping 1751 Haste on one item). then you have the 2 pc setbonus, but it does nothing for you as a tank. perfect
the retribution chest is usually taken by prot tanks (if they can afford it to get it), but the helm is only used if you ignore the 2pc prot bonus (which is, if used, helm+shoulders, but again, this is lower haste than retribution helm + rnd boss drop shoulders.)