I really don't have a preference. I mean, I like hearing the original voices, but sometimes it's easier to tell who's talking to whom when you're listening to a dub. Also, I've run into some really bad subs. Granted, most were fan subs done buy people who had little to no grasp of spelling and sentence structure, but a few were supposidely proffessionally done. (Take the Hong Kong sub of YuYu Hakusho for an example.)
Dubs can be really bad, too. Tokyo MewMew turned into MewMew Power with some cut rate awful editing in parts is the first one that comes to mind. (4Kids really likes to throw out boners.) And yet sometimes they can actually turn out better than the original. Personal opinion here. I **much** prefer John Burgmeier as Kurama to Ogata-san. Not that I have anything against her, mind. She has a great singing voice and I love the theme songs she did for him. But I really think she was the wrong choice to voice him. (Kurama sounds like a 12-year-old in the original! The guy's supposed to be sixteen!)
It really all depends.