Trying to compare survivability between DRK and PLD

[Cabalabob]

Short answer, PLD is better.

Long answer, PLD has clemency which is just the outright most broken heal for healing other party members in low man content, PLD also brings cover which will allow you to protect a party member from mechanics they may not survive on their own, examples such as exploding towers that are meant to be taken by 8 people will deal massive damage and usually apply vun up that most people won’t survive, but a PLD will and they can take it for others too.

While WAR can certainly keep up with PLD on the healing front, no other tank, especially DRK has any answer to cover. PLD also has extra party wide mitigation compared to DRK and it applies to all damage types where DRK only has magic protection.

PLDs only downside is it brings less damage and it gives up even more of that damage to heal others. So there’s the potential to not meet enrages if your dps isn’t good enough where a WAR might.

You may notice I mentioned WAR a lot and not DRK and to be frank, that’s because DRK pretty much isn’t in the conversation. PLD and WAR are the tanks with the sustainability, DRK and to a lesser extent GNB simply can’t keep up with them. So unless you are doing very low level content to the point you don’t need any healing those two tanks are going to be less than ideal.

[LoadedVirus]

Dark Knight simply can't keep up in solo content compared to the other 3 tanks. You CAN solo with Dark Knight however making mistakes pretty much means death while the other 3 can just heal through it, Blackest Night while really good in theory does not keep you alive anywhere near as well as a good heal would. I would rank all the tanks when it comes to soloing like this WAR -> PAL -> GUN ----------> DRK.

[Oizen]

Where DRK struggles is that of the three types of defensives (Mitigation, Shields, and Regen), DRK only really has access to two, being Shields and Mitigation.
TBN is the fattest shield a tank can give itself, and its cooldown is very short, but its not constantly up, and because it eats into your resources, and is a damage loss if used too much, its not something you can use too much.

The only single target regen DRK has is an incredibly small 300 cure potency on the last attack of its single target gcd rotation. This works well enough in low level content, but around HW-SB levels it becomes basically irrelevant.
So if you're not doing rotation in favor of Bloodspiller spam, this means DRK has no single target heals until you get back to it, which can honestly be about 20 seconds.
AOE wise its not much better, Abyssal drain is honestly terrible. 200 cure potency per target sounds nice, but its 60s CD makes it basically unnoticable, and theres nothing beyond this.

(these numbers are laughably small compared to basically any other tank)

This results in any damage DRK taking basically sticking, it is very hard for a DRK to refill its health once damage is taken, and unfortunately with lower health Mitigation is a lot less effective.

Now DRK's mitigations are either Busted or mediocre depending on the content. If theres a lot of Magic damage, DRK is incredibly sturdy, if its more on the physical side, then DRK is significantly more frail.
This basically comes down to the skill Dark Mind make or breaking DRK. Its either rampart with a shorter cooldown, which is insane, or absolutely useless.

TBN does not hold up to the other 3 Tank short mits, Oblation kinda evens this out, but then thats two mitigations you have to cast to equal another Tank's singular mit. In tight situations where you have to react fast to save somone with your skills, DRK once again comes up short.

You can make DRK solos/duos work, but the job is incredibly selfish to no real benefit of your friend, especially when you find out its really not even that good at doing damage without heavily relying on raid buffs, which obviously wont be that common in duo/solo content.

EW DRK is in a really weird spot right now, if you're not specifically in love with its play style, or doing savage content I wouldn't recommend it. WAR right now is the tank I'd say is best job at literally everything under the sun.

PLD has its own problems too, it gets its healing late, its damage isn't all that good, but in terms of party utility and defensives its a solid choice.