Well I can sympathize with your situation due to having gone through it myself. I am also looking into international law, so I assume we are looking at a lot of the same factors.
I am not sure if you have visited GW, but that would probably be a good idea. GW's campus has a lot more in common with BU than with BC as far as location goes, but GW's buildings are much nicer than BU's and closer to BC level. GW is in walking distance to pretty much all the major sights, which is impressive. I don't place a lot of stock in talking to people while on campus, since it can be hit or miss and probably not something you should base your decision on. Someone might just be having a bad day. However, both BC and GW seemed to have friendly environments.
As for how I decided on BC, they gave me more money than GW, I live in Boston now and I like the city, and living in Boston after graduation is fine with me. Although I'm interested in Int. Law, I sort of discounted that out of the equation and looked at location instead. I might not be as interested in Int. Law as I thought and most likely I will be a generalist anyways. GW obviously has the upper hand for government work/internships due to location, but I wasn't sure it would make that much of a difference to leave Boston, especially since I'd like to work at a private firm.
So when I went to go visit GW having seen BC, I suppose they were about tied, with BC having the edge due to the ease of transition. I pretty much would have had to LOVE GW's campus to have left Boston, and that didn't happen.
Ok, this was a little long winded but I hope it helps in some way.