Dubbed primarily, but I have rules. I'm not a sub-hater the way I used to be.
I'll for sure watch it dubbed without changing my mind if:
1) The dub is good enough for me and/or better than the original Japanese (Believe it or not, it's possible that Glorious Nippon can be trumped every once in a while. Cowboy Bebop, Haruhi Suzumiya, FLCL, Evangelion, Samurai Champloo, Trigun, Outlaw Star, Cromartie High School)
2) I grew up with the dub and so have nostalgia for it (Rurouni Kenshin, Yu-yu Hakusho, Great Teacher Onizuka)
3) The show takes place outside of Japan (Baccano! Fullmetal Alchemist)
4) The main cast isn't Japanese, therefore it makes no sense to hear it coming out of their mouths (Black Lagoon, Baccano!, Fullmetal Alchemist)
I'll for sure watch it subbed without changing my mind if:
1) The dub sucks (Irresponsible Captain Tylor, countless others)
2) The show's too culturally Japanese to make sense of in English (Sayonara Zetsubou Sensei)
3) I simply can't wait for the dub, even if the dub is good (The Disappearance of Haruhi Suzumiya.)
4) I start the show subbed, and so can't be arsed to change it back now that I've gotten used to the voices (Fate/Stay Night)
5) The show isn't getting licensed (Detroit Metal City, Ore no imouto ga Konnani Kawaii wake ga nai)