PvP Materia?

Are they worth getting for PvP? How much do they actually help with resistance? I thought about getting some if I had spare marks to use between a gap of ranks where I no longer need armor. Like rank 27-30? I'm currently rank 17 and I have all my i70 PvP stuff, don't see the PvP materia worth it at this rank.

I think 1 ailment resistance is equivalent to 1% reduction in duration.

10.5 seconds of stun with..

2 stun resist (1 materia, 375 marks): 10.29s duration - 0.21s reduced
10 stun resist (5 materia, 3750 marks): 9.45s duration - 1.05s reduced
26 stun resist (13 materia, 4875 marks): 7.77s duration - 2.73s reduced

And so on. The gains are so minuscule when compounded by the mark cost that could be saved towards better gear. Everyone leveling up is scraping together marks to buy gear and probably isn't stacking resist materia to produce a noticeable effect. I wouldn't bother until you are at the very end of the gear scale and have nothing left to spend on, at which point the investment can be seen as worthwhile since you won't replace that gear for a long time.

With how much they've increased mark accrual you may find excess in between equipment tiers, in that case by all means buy a few to slot into your new pieces.

What about non PvP materia for a tank? WAR/PLD? Thanks Brahms, I think I'll do exactly that. I was just curious to see if there were any real benefits from it and that just shows me only at end tier there is.

Stack your preferred secondaries and complement it with ailment materia if you're inclined. No real guide needed as you're pigeonholed into socketing certain materia on equipment thanks to meld caps (chest with high determination: only critical hit and skill speed can be socketed further of your relevant secondaries).

As far as ailment materia go it comes down to preference again, though a large number of them are utterly worthless (silence, poison; even slow and blind). For tanks the ones you deal with on a regular basis are sleep, bind and sometimes heavy. Stun cycles from an enemy tank can put you out of the game too, but it's not as critical as it is on a healer who needs to be able to cast, for example.

Marauders/Warriors have different priorities and sometimes have to purge a Tri-disaster/Fluid Aura bind, so bind resist is much more valuable to them. Meanwhile Paladins have Tempered Will to escape and the value of bind materia is greatly diminished. I personally stack sleep resist because enemy teams make it their express goal to keep me slept and controlled. Due to the high duration of sleep you gain greater returns from a single point of resistance (0.45s per materia if we were to discount the first sleep being cleansed and assume I am sitting in a 15s/7.5s sleep cycle afterwards). Second in priority queue would be stun.

It's all up to what you value and want to deal with less of, really.