It depends on the anime and the voice cast for me, really. Some anime I cannot bear to watch dubbed, because the voice acting is either absolutely appalling or really doesn't do the show justice. Having watched Sengoku Basara both subbed and dubbed, I will never watch it dubbed again - the voice actors just can't do justice to the over-the-top nature of the show. Similarly, Death Note loses a great deal of its drama in the dubbed version. But, that said, I would never watch shows like FMA Brotherhood, Noein, any Gundam series or Ghost in the Shell: Stand Alone Complex with subs, because the voice cast is absolutely fantastic and makes the show so much better.
As a rule, I tend to see who is voicing whom in the English release before I decide whether or not I'll watch it dubbed. I have no real preference between the two formats, as I've long since gotten used to reading subtitles when watching a show, and I'd miss out on a lot of fantastic anime if I only watched dubbed releases.