PLD and its not even close. PLD has group mitigation with passage of arms. Group shields and regen. Clemency and intervention. DRK has TBN and oblation and dark missionary which provide no healing and only mitigation, and dark missionary only covers magical attacks.

If you want the cool factor go with DRK and sacrifice group utility, if your party needs extra help with heals and shields and mitigation go with PLD. For dungeons at early game you are better off going with PLD.