I'm in the same boat! I got accepted at SAIS (DC campus) with no aid and Fletcher with a nice scholarship. Of course, I'm leaning towards Fletcher to save some money. My end goal, though, is to work in Washington, DC, so I would rather go to SAIS, all other things being equal.
I've been racking my brains trying to figure out what to do, but, depending on what I hear back from MSFS, I'll probably go to Fletcher. Many Fletcher alumni go on to top jobs in Washington, and I don't think a SAIS education is worth the extra debt. Boston may not be the international relations "hub" that Washington and DC are, but Boston offers a lot of academic and research opportunities (Harvard, Tufts, MIT, etc), if that's your thing. I'm also very turned off by SAIS's lack of funding. I understand that Fletcher awards scholarships to 90% of its students, while almost nobody at SAIS gets money. I just consider that rude haha.
I'd be very interested to hear everyone's final decision and how you came about it!