I generally prefer subs. I've heard the dubs for Cowboy Bebop and Samurai Champloo, and I thought they were awful. The dubs for Outlaw Star, however, were alright, and it's currently the only anime I could bear to watch with dubs.

In terms of films, I've seen most of the [Miyazaki] Studio Ghibli films in both English and Japanese, and I think the Japanese ones are better, though the English voice-acting is of a good quality (mainly because a lot of them are big-name actors).