Pretty much what Piana said. DA is pretty much the best "starting off in a new server" class since you don't need pots, they're pretty strong on their own, and have a great link skill and character card.
DA as mentioned above is a great starting class. DRK is, however, a very good class for the long haul and also unfunded. You have high damage percent with Gungnir and you still scale extremely well with funds due to the crazy amount of final damage you get from cross surge. In terms of overall bossing, not DPM, they are both similar in capability, but I would give that to DA as they have more waveclear and tank better than Drk.
Dark knight wins in damage for the long haul, but DA is the better bosser. Both are equal when training.