In my opinion i think .hack//sign is better than .hack//dusk. Sign just had more plot and mystery to it i think to make it more exciting. But i also enjoyed Dusk alot too.
What's Dusk? -_-
Legend of the Twilight (LotT) and Tasogare No Udewa Densetsu (Udeden) those are the only two acceptable names. Dusk was crappy translation by a certain fansubbing group.
Hmm? no DUSK, is a poor translation of LotT Anime, it was given that name by fan dubber's and has sort of stuck to it... though I suppose if not many people knew what it was, then I suppose the screw up isn't as bad as some people thought.
Now I only read the manga but I think SIGN is better since the plot was better, yuri, and some more memorable characters. But in Dusk's favor Zefie and Ouka are cool.