If you intend to mting raids, it'd seems PLD will remain as the most stable tank.

However, DRK is pretty good if you are planning do dungeon speed runs (Unleash...salted earth...and maybe abyssal drain if you have the mp *cough* blood price *cough cough*)

No one can really say for sure yet until people actually try using DRK for the new raids to come. We will see soon enough.

I won't say that you made a wrong choice, both have their uses in different scenarios. Dark Knight is simply not a proven tool yet, too early to judge anything.