I usually stick with subs, especially if the dub version contains heavy edits or the show relies heavily on Japanese idioms or wordplay. I find the quality of voice acting is a lot more variable in dubs, and they often overact or miss the mark on the tone of scenes. Some dubs are excellent though, like Cowboy Bebop and a few of the studio Ghibli movies. There's even a few dubs I'd consider to be better than the original version, namely Baccano! and Samurai Pizza Cats.