You want to go Resolute then o,o Mystic is squishy af (set itself offers 0 survivability against a proper defence, ull be ccd and rekt and 100% reliant on CLs, everything will crit you considering the crit rate vs resistance ratio atm) and dps is too proc reliant, Ruin has no dps whatsoever. If you want a good carrier build and be able to kill u want Resolute. If you are gonna mainly carry then you want either full Ruin OC or 4 piece Ruin OC with 95H Body and Shield, which is the tankiest build in Aika, but you wont kill anyone that matters with full carrier build, though u will be the tankiest person in the game
If you only have Ruin or Mystic as options, just know that Mystic is not a good carrier set against proper defenses as I said earlier. As long as you are a high-ish + Pally you can TD with any 90-95 set with Wards and Defiance etc but only against weak or low number defenses because u will have minimal to no cc on you....that means you are completely relying on your class, the set is doing absolutely nothing for you as a "carrier". When you are cc'd a lot, as you will be against big defenses, you will be completely reliant on escort and cls, the set will offer no help for the cls either since u will take a ton of damage lol. With that said, Mystic is much more helpful during defense then Ruin, during defense Ruin only helps if it OC Stuns someone. Mystic at low/decent + armor pieces can be helpful during defense and to FF cls during raid/defense, but at low/decent + its trash as a carrier set when it counts. You will have to favor carrying or defence/pve and decide for yourself. Personally I can never recommend Ruin on pally unless ur only gonna carry, I can tell you though, if you decide Ruin then go full Con, hybrid or even full STR wont help Ruin dps much (I have tested Ruin OC with full str build) Resolute is the only set that can do both effectively, allows for various builds from just switching accessory sets and mount.