When should I choose +health modifiers over +armor modifiers and vice versa?
I sometimes find myself facing a tough decision between a piece of armor that offers +10 to armor and a piece that offers +50 to health. The +50 health is great, but will that +10 armor ultimately save 50 health or more in the long run? Is there a way to calculate at least a semi-accurate figure to help determine which is the better modifier?
The most common way of calculating this is calculating EHP(Effective HP): The rule is simple. You basically calculate the amount of damage from mobs it takes to bring your HP to 0. Say your HP is 500, and mobs hit you for 50 dmg and you have 0 armor. Then it takes 10 hits to get you down to 0. If you choose +50 hp it means it will take 11 hits to take you down.(EHP = 550) If you choose +10armor it means each hit will take only 40 dmg which means it will take 500/40 which is 13 hits to take ur hp below 0.(EHP= 50 *13 = 650)
+HP usually can be good in areas where the mobs are heavy hitters and will take you out in 1-3 hits. The bane of an Elite Hardcore player is a crit damage from a heavy hitting mob which usually kills a player with low HP. So some higher difficulty players(Especially Hardcore players) swear by +HP just to have some insurance against damage spikes from boss crits or mob crits as armor will not help you much here(%Dmg reduction does though!).
So as you see it is situational. While +Armor is usually better in EHP with low hitting mobs, +HP could save you from damage spikes from boss crits etc.
Additionally I recommend you to min/max your equips for certain stats (Only +HP or Only +Armor) cause it adds up and thats when it starts to be really effective. Individual equips hardly ever grant any major benefits, but I guess you already knew that.