What about a potent "killer" effect to each? Maybe like PLD get Undead Killer which improves damage, defense, aggro, and cure damage vs undead, and WAR gets like, I dunno, Beast killer where their DEF, ATK, Aggro, and uhh....whatever else.

I'm not talking weaksauce like XI, but something that makes people think, "Gee, we're gonna be fighting Odin (he's death/undead right?) so we can go WAR or PLD." PLD might hold an edge over the WAR. Or you fight Chimera which is a beast, so WAR will have an easier time with Chimera.

That's not to say a PLD can't tank anything but Undead, or WAR can't tank anything but beasts. I dunno...