In my opinion, because I play DRK and WAR extensively, I think they're both equal in dungeons when it comes to clearing trash. In terms of soloing bosses, WAR is better but DRK can be surprisingly resilient when you neglect bloodspiller in favor of accessing your SE heal as much as possible while spamming TBN too.

Now, despite the fact I think they're both equal, I would recommend DRK because WAR's style of using nascent flash tends to scare a lot of healers-- Healers seeing a yo-yoing HP bar tend to turn into curebots. Not always, and not with healers who can understand what's going on, but that's not something that happens with DRK.