Just abut every major tank buster in the game uses a mixed type of dmg mod, wheres its both phys and magic, save for the purely magical ones in some fights that you listed. Personally I see both tanks being just fine. Obviously certain classes excel at certain fights, but really, it comes down to team comp. Its silly to have a drk and a mnk in the same party, when you can change your mnk to a nin or change your drk to a pld.