New(b) PLD

[SpookyGhost]

You don't need either of them if you're just farming Wanderer's. Or AK really. They're helpful (Mercy adds to DPS, Stoneskin gives healers even more breathing room if there's a part in a fight where you can actually cast it) but they're not exactly something that should be prioritized for farming. Gear is more important, so if you're still farming phil then keep doing that with what you've got. Eventually you may want to get Mercy Stroke at least as you progress and if you haven't done Titan then Stoneskin is helpful for that fight, but for now you can just chill and keep farming tomes.

Edit: Though Stoneskin is more useful in fights where you can cast it (Titan HM~), Mercy Stroke is something that is universally useful whereas SS is just a useful quality of life tool to have in certain instances. There's no fight where either will really change the course of events, but SS can slightly help if your healers aren't on the ball for Titan. So basically, get them eventually but gear is more important.