I have one passive choice for defense; which one should I pick?
My first two passives are for my build, adding crit chance, so the third needs to be something defensive.
Tough as Nails, Nerves of Steel, and Relentless come to mind. I know how to determine the highest armor bonus from the first 2 options, i.e. whether the additional armor from vitality or a 25% increase of current armor is greater, but I don't know how the armor bonus compares to relentless. I'm using Frenzy, so Life on Hit will be high, at least 700, and I'll be using a shield. Impunity as well.
Not sure the best route to go. I don't want to hover around 20% life all the time, but I don't want to die to lag or not have enough mitigation from armor...
Re: I have one passive choice for defense; which one should I pick?
Whichever gives you the most benefit - if tough as nails/nerves of steel raises your DR from, say, 50% to 65% or even 60%, I would say it is superior to superstition. Non-elemental melee is also some of the hardest hitting in the game - in just A3, swift skull cleavers, demonic tremors, golgors, packs of enraged blood-clan and even fallen overseers are likely the hardest mobs you will face, so elemental resist is irrelevant there.