Majority of the bosses you can tank as a FM without many problems, the only ones that I can think of that you can't tank are open world raid bosses.
Blade & Soul: Evade or Block for FM
You are probably going to say this is not helpful at all, but the most helpful advice I can give you in regards to survivability as FM is to learn how to tank the boss yourself. Majority of the bosses you can tank as a FM without many problems, the only ones that I can think of that you can't tank are open world raid bosses.
You are FM, you are going to pull aggro, if you are even half decent at your class, it's a given. Instead of brute forcing and relying on RNG, because that's what evades / blocks are, you really should take your own survival into your own hands.
Now onto something more relevant to your original question, which defensive stat you should choose. Short answer, whichever one that gives you the most effective health point (EHP). Your question is actually quite complex fundamentally as you will see below, there isn't a clear cut "this is what you go for, all the time" thing, which is unfortunate.
*Warning, numbers below, you have been warned!*
I haven't done calculations for evade and block but for defence the formula is:
Defence % = (94.9 x Defence rating) / (4707 + Defence rating)
*Though you don't really need to know the formula as you could just swap the equipment and read it off your char sheet. Knowing the formula just enables you to be able to do the calculation without having the equipment, so you know if you want to spend the time to farm for it or not.
Basically, to pick from HP, Defence, evade, block, you need to know your current EHP, and then calculate the delta to that EHP when you swap stats around. Without the evade and block formula I can't give you an exact equation but here is an example.
EHP = HP / 100% - % Damage reduction
To add in block
EHPb = HP / 100% - % Damage reduction / (100% - % Chance to block * % Block reduction)
Now add in evade
EHPbe = HP / 100% - % Damage reduction / (100% - % Chance to block * % Block reduction) / (100% - % Damage reduction)
Using my own character with 63141 HP, 26.37% damage reduction, 29.84% block rate, 39.49% block reduction, 21.22% evasion. Her EHPbe would be
63141 / (1 - 0.2637) / (1 - 0.2984 * 0.3949) / (1 - 0.2122) = 123,393.54
Let's say I can reduce my HP by 1,000 and increase my defense by 1%, should I do that?
62141 / (1 - 0.2737) / (1 - 0.2984 * 0.3949) / (1 - 0.2122) = 123,094.65
The answer is no I shouldn't, as this change would give me 298.89 EHPbe less than my previous configuration.
Of course, this is a pretty vacuum calculation, without taking into consideration class specific skills or damages that ignores defense, can't be blocked / evaded. In order to REALLY know what's going on, we need to model this with a simulation. And also the way the game decides if it's a hit, block or evade will play an important factor as well.
All in all, you actually asked a question that is quite complex in nature, as there isn't a "formula" per se to tell you exactly what you should or should not do. But the one I provided should give you an idea of what you want to do.
Blade & Soul Dungeons Suggestion
Even at the very start with the Moonwater Recipes, drops were based randomly off RNG. And that's perfectly fine, I don't mind that.
Blade & Soul Desolate Tomb rewards of 4-man version
The loot of 4-man Desolate looks very unrewarding compared to other easier 4-man dungeon, while people can still spam 6-man to get the legendary necklace with less efforts.
Stop boosting BD in Blade & Soul
"Des never being able to die" the class with the least iframes and that doesnt even bring party iframes.
Blade & Soul Big Dungeon Patterns
The big dungeon pattern. The next big dungeon which should be at December or early next year would probably be 700ap for above min requirement