Earplugs! Earplugs are the clear winner of this contest. I did alternate between earplugs and music for equal numbers of work hours but this isn't a very scientific experiment for loads of reasons, and I know that. Sorry, I didn't track the progress in the sidebar as promised, because my HTML skills are pretty limited.
It was hard to quantify productivity in terms other than words/hour. But I not only wrote more words with earplugs in my ears (almost 1,000 more over the two weeks) than with headphones on, I also had a major breakthrough on my manuscript with earplugs in. All of this despite my bias towards thinking that music made me more productive.
Maybe it keeps me company while I'm working, I don't know, but the truth is, I think I am addicted to music. I'm not sure that even after such definitive results, I can switch entirely over to earplugs. I will have to wean myself off of working to music :(