go to the interview. why wouldnt you? unless you have something against BU or paying for the ticket would mean the only meal you would eat for the next few days would be the interview day lunch, then I see no reason not to go. BU seems like a great school, especially if youre into research.
First of all, congrats on your acceptance to GW! Those seem to be even harder than usual to come by this year. I would vote for going to the BU interview, maybe you would really like it, and you never know unless you go there and check it out. Plus, GW needs lots of waitlist movement. Lots. Go to the BU interview! 😉