You're mistaken here! Even if resistance triggers, the Warlock will still deal Chaos damage. Don't forget about his Irresistible Magic.
Dungeon is my weakness...
Mathematically, the skills are equivalent. Therefore, the conclusion is that resistance should be taken against the Warlock if we are focusing on defense. Luck and deflection should be taken against the Warlock in any case.
But plague deflected once...
Except the ones we've made