I find that even the very BEST of dubs simply lack the umph of the original. There are very few cases where the reverse is true. Dubs are usually done relatively hastily and with inferior voice actors cast and directed by often inferior directors. Even the Mononoke English dub, with its all-star cast, just doesn't have the panache of the original seiyuu work. Some get by better than others, like Eboshi and the straight-man Ashitaka, but on the whole, it lacks the cohesion of the original. Even a great actor like Billy Bob Thornton just can't really fit squeeze into that little monk's character. It's like wearing somebody else's clothes, and you can tell where it fits and where it doesn't.

Now a dub, even a mediocre one, is probably better than nothing at all, but whenever I have a choice, I pick subs for the above reasons.