It's probably best to bring both a NIN and a PLD to each mission 40 and up. 2 tanks means both have to die before anyone else, and no matter which tank is better, you have it. Plus a NIN and PLD combo does as much damage as tank+DD, and tanks better than a tank+DD combo too. (except maybe snoll tzar.)
Though if you gotta pick just one, probably PLD. There's a lot of AOE's, multihits, and moves that simply bypass shadows in COP missions. (And terror and petrification outright kills NIN.)
Old professor. Either one really. PLD can shield bash spells, NIN can absorb them with shadows. NIN can Ni nuke, PLD can cure, about equal. He's a pussey so long as you can handle the two students.
Promy Vhazl. Lots of AOE's from the bosses, a couple multihits, and the fact that there's multiple enemies puts PLD ahead. Plus invincible's a life saver.
Three forks is up in the air due to stratagy. Mithran trackers AOE/astral flow killed= PLD for invincible-> aoe kill, one at a time it's a tie. Snoll it can be either, PLD to hold hate with invincible so you can cram more DD in, or NIN so the tank does some damage, too. Depends how you're killing it. Moblins, everyone soloing/kiting one NIN wins, supertanking or kiting all of them and killing one by one PLD wins. (well BST wins for kiting, but meh.) All basicly depends on your stratagy.
One to be feared, preferably both, since they're about equal on mammets, and one is better on one weapon, the other better for the other. Again, up to your stratagy.
Maybe if we wait long enough, he'll tell us about how he walked barefoot uphill through snow both ways in Uleregand and defeated the evil Snoll Tzar with nothing but a stack of pebbles. Men were men back then. Mithra were men, too, but they don't talk about that.
Mellowy is awesome, now.
I'm sorry. My hoovercraft is full of Summoners. As soon as I claim a thread, they just flood out and cover the whole place. Slippery suckers, those Summoners.