For the PLD:
- Tank stance = enmity bonus, accuracy bonus, effective health bonus via reducing all incoming damage.
- DPS stance = extra damage on auto-attacks. That's all. Yes, many people forget this, but Sword Oath only ever benefits your auto-attacks. It doesn't magically make PLD skills hit harder, not for the tiny least bit.
Going into either stance incurs the GCD. That's why PLDs don't really do stance dance (they do switch stances when there's no incoming damage, but nothing close to the stance dance healers do).
What would solve the STR vs VIT debate is to redesign it so that it's not a trade-off between the two stats. Imagine if Crusader does not swap your MND and INT, but adds a flat say 20% damage to your spells in exchange for 20% less healing done, giving you healers who swear by going full INT on accessories.
"I don't need that extra healing power because it'd all end up as overheal anyway. If anyone needs that to stay alive, they should just get gud. Any healer going full MND is just dragging out the fight and are just noobs."
The STR/VIT debate isn't about stances but about the relative value of the two stats. Generally speaking, it's the old school "tank=absolute maximum survivability in worse case scenario" and the new generation "tank=enough survivability then maximum dps". Just like how healers can dps, the game gives significant freedom to how one plays the tanking and healing roles.