Which do you prefer?
I'd have to go with hack dusk
IMO these 2 animes can be categorized like this:
.HACK//SIGN - More Mature anime with less action a nice story but makes you wish for more
.HACK//DUST - More Childish anime but with lots of action and also with a nice story.
I recommend people to see both but if you have to choose one of them, pick the one that suits best your taste.