The effectiveness of armor and resistances depends on the level of the monster that is causing damage to the hero. According to one post on the Diablo III forums, the formula for armor is
DR from Armor = Armor / (Armor+(50*mLvl))
Now, I would assume that for most of the game the monsters are close in level to the player, at least while the player is still gaining levels. But how do the monster levels far on Inferno difficulty? The player is fixed at level 60 there, are the monsters also fixed to this level or does their level increase further in higher acts?
The reason I'm interested in this specifically is that a further increasing monster level would make armor and resistances less effective compared to just increasing the HP by stacking vitality. Knowing the approximate monster level is necessary to calculate whether adding more HP or more armor/resistances is more effective at a specific point.