When I watch it online, I prefer subs. But since I watch Toonami, it's not like I have a choice, but I don't mind. Otherwise I do prefer the English dub of DBZ to the original, I can't stand how Goku sounds like a kid and acts like an idiot in the original. The only other time I put on the dub is if I don't feel like reading the subs/ to tired to read them.
Subs are usually better since they honestly feel right. They put more emotion and thought into their voice acting. Some dubbed shows are actually better though like DBZ and Yu Yu Hakusho.