I second what Ryoutarou said about the Fruits Basket dub. It was an excellent dub that one.
Anyway, I usually end up watching subs, simply because I don't have a wait a week in between to watch the episodes, and that I can watch when it's convinient to me, and not Cartoon Network.
But I don't have a preference. If the dub or sub is good, then I don't particularly mind watching it in either Japanese or English. (Although if the dub is painful and poorly done, that's a different story.) However, usually the case is, if I watch it first in Japanese, then I'll prefer it in Japanese, and if I watch it first in English, I'll prefer it in English. I like voices that I'm accustomed to. xD