The key differences are that DRK is a bit more complex, but also WAY squishier in Dungeons (or anywhere with a lot of Physical Damage).
Drk is especially fragile below lvl 70 because it doesn't have The Blackest Night yet.

DRK has only 1 self heal, and it needs 13 targets to heal 50% HP (LOL)
DRK has 2 mitigation buttons (Dark Mind and Dark Missionary) that are useless on physical damage which is far more common than magic damage in dungeons.
If you want to see what attacks are Magic, you have to violate the ToS to be 100% certain.
Also, Bloodwhetting >>> The Darkest Night

Damage wise, DRK is higher, but you gotta mash a lot of buttons every 2 minutes. WAR is a bit weaker, but not as mashy.
The damage difference isn't really too much to be a concern though as I see it.