Definitely Zoroark. Although Dusknoir may seem appealing due to it's higher defenses, it has a horrible offensive typing, since it's a Ghost type without a good physical STAB, and only a mediocre base 100 attack. Trick Room may be useful, but there's a lot better Trick Room users in the game, like Carbink, for example. Most of the time you'll be missing some KO's with Dusknoir. On the other hand, Zoroark has a great offensive movepool on the physical and the special side, has an option to boost itself via Nasty Plot, and it has Illusion to fool the AI. Also, it evolves earlier than Dusknoir, which needs a Link Stone and a Reaper Cloth, which is gotten around the 14th gym, I think.
TL;DR: Zoroark>Dusknoir. Better offensive stats and movepool