Prescence or Toughness. Personally, I would go for Prescence because while it gives less health than toughness, it gives a boost to leadership which boosts your teammates stats. Btw, if you want to tank, I would personally go sword and board because shields give you crit immunity. Also, Rime Vail, a spell you gain by giving Cait the Rimebound Grimoire is one of the most useful powers for a tank.