It depends on values you have and what opponent is using. Toughness scales well if you already got a lot of armor+hp. Armor is always better if you have low armor+hp. Toughness vs non penetrating weapon provides at least 9% raw damage reduction, but if opponent is using plasma or power weapon, toughness is wasted. So its situational. If you're stacking a tank built, both are amazing, if you have base stats and wonder what to add, its a matter of preference.
Lets just break down what the stats mean because which is better varies on the rest of your stats. Health is health, it does not regenerate but it can be healed. Armor is like a "shield" its just another HP pool but it does regenerate over time. It can't be healed or repaired outside of its normal regeneration. Toughness is "mitigation" or basically damage reduction. I don't remember the exact numbers but I think its like 10 toughness is like 5.something% mitigation. What does this mean? It means if you have very low health and armor values then more health or armor is better. But if you already have lots of health/armor (relatively speaking naturally) then toughness will give you better returns. To make it really really simple, just focus on Health/Armor, and get your toughness (if any) from items that give both like upgarded armor or something similar.
I've not seen it said either way exactly what sort of damage nid's do, if its low penetration then toughness will help if it is high penetration then it will be a wasted stat In PvP i'd go for armor as it applies to all so will more likely be useful but it would depend on how you are playing, if you play chaos and only play in premades then toughness + squad leader with damage reduction banner might work out better
It is always a hard question to answer. Toughness is in all cases is the cheapest defensive stat and benefits you most if you plan to go heavily into tankyness. You get the most out of the LP (usually 10 toughness are worth around 50 LP) invested. Toughness compared to health and armor brings 10 toughness which translate to 14-18 Armor/Health (depending on your total numbers). Health and Armor basically are the same thing, Health can be healed (which is an advantage if you run with a dedicated healer) while armor can recharge on it's own (better if you go with a recharge build on veterans or run around solo alot). LP wise Health is always cheaper compared to Armor. For Comparison 30 Health cost ~180 LP, 30 Armor 210 LP. Since you can not really stack one without the other i would suggest the following: Chestitem is always the priority as it has the biggest impact on it's own (because there are usually no other alternatives bringing as much defensive stats on 1 item). In Wargear slot 1 I would always prefer toughness > health > armor. For trinkets I would personally go with health > armor but the amount there is usually (unless you are right at a threshold and need this 1-2 extra hp to survive an additional bullet) insignificant. For PVE i would go with toughness and health > armor. Since you have a healer, especially if its a warlock or sorcerer.
I crunched some numbers and it is basically as Katitof says. 320EHP @140 Toughness 350EHP @ 120 Toughness You can take one more Power Axe Fast Attack without dying with the improved Toughness. All other TTKs are identical. 300EHP @ 120 Toughness 330EHP @ 100 Toughness Ranged TTKs are all the same, but melee favours the bonus armour over the bonus Toughness. If you can get your Toughness over 120, take that. If you can't, take armour. Edit: The question is, however, does 20 Toughness cost the same LP as 30 Armour?
I was kinda thinking that, yeah. So how much Armour do you get for 20 Toughness? I'm not at home right now, so I can't check.
