I have no numbers to back it up, but I would think that interviewing in person would be preferable. Shows that you're serious about the program and allows for a more dynamic interview. Also avoids any technical problems (I don't know about you, but skype ALWAYS lags for me after about 20min of talking...) You also get to see tha facilities.
I would also consider whether you feel that your portfolio needs/could benefit from additional in-person explanation. Considering that you're applying for Graphic Design, it might not need any more context (compared with a portofilo in say, Sculpture/New Media etc.).
Plus, it's a helluva lot harder to turn somebody away when you've spent time with them, had lunch with them, gotten to know them as a person and an artist.
That being said, it is a huge amount of money AND time, and there's not guarantee that appearing in person will get you in.
Don't know if that helped... still doesn't fully address the question.
Does anybody have any stats that might address this question?