Just signed up to say I'm in the exact same boat you were a year ago: rejected by Tufts, accepted by Deis, NIU, and GSU so far, waiting on UW-M. It sounds like I'm a little more fanboyish about Brandeis' faculty than you are/were, but I'm not intimately familiar with NIU's work.
In case anyone else is browsing here cause they're in the same boat (all ten/fifteen of us or whatever), I figured I'd share my recent reasoning.
1. One thing to clarify about Brandeis. I received $10k merit- and $10k need-based scholarships from there (w/ that $3k per semester TA-ship). Some people have been saying this is "50% remission," but that's misleading. Others might have been receiving more in aid and been referring to that, but I've seem some call this $20k package 50%; since $10k is the limit for need-based aid, I think I received the highest package; of course, might be wrong.
However, $20k is 50% of *one-year* of full-time attendance ($47k). So if you struggle to fit 9 B+ classes (one which is the thesis) within the first year (which is almost guaranteed), it won't be close to 50%.
That said, I've noticed a kind of loop-hole in the funding structure (and mention this to calm down anyone else reading who feels as insecure as I): at 9 courses over three/four semesters, you're better off doing part-time, per-course payments of ~3 courses for the first two semesters to fulfill the residency requirement ($17k+$17k=$34k vs. $47k), and then move down to the post-residency fee ($6k per semester). This will indeed make that $20k package 50%, but only for part-time students finishing in three semesters. Since it's probably a bad idea to take two other courses while you write your thesis (and probably working a part-time job), you'll maybe want to pay an extra $6k for a fourth semester, taking the thesis class alone or with at most one additional course.
To be clear, if you do the full-time tuition year at 4 courses a semester, expect that you will need to take at least one post-residency semester for the thesis (9th course) anyways. Doing part-time--probably no matter how you balance the course-loads out--will save you $10k. I'm thinking--if I go to Brandeis--it would be best to do 3, 4, 2, arriving at the initial ~$47k over a more manageable three semesters.
1.5 BUT: Boston is hella-expensive, so this would require taking a part-time job those three semesters just for room-and-board. NIU, on the other hand, would effectively be paying me to study full-time there. (I figure thrifty living + petty savings + summer-jobs could make do with the $12k yearly stipend.)
2. A note on assistantships: I was accepted to GSU with the full assistantship package. At first, I thought of it as potentially great work experience, getting to teach 5 easy classes over a year (at $10k total). But now I think it's a straight curse. That's five classes to teach while you not only finish up your master's course-load but also have to complete your thesis. Again, compare that to NIU (or Brandeis), where the assistantship is more traditional: background activities, grading papers, etc. For NIU, you can do that for better pay and afford rent while finishing your studies/thesis; for Brandeis, you'd have to take some menial part-time job for rent, but it probably would not be as worrisome as teaching 5 undergraduate courses. (I actually still waiver on this point. The experience might be worth it, and I assume grad students at GSU are helped a lot in terms of course planning and grading (from younger TAs).)
3. On faculty and AOI: I'm sure everyone involved already knows, but it really is remarkable how distinct the AOIs are for these schools despite being just terminal masters. GSU is almost exclusively a social-political/ancient oriented program, to the point where I'm already close to turning down the offer -- I'm a language/epistemology/metaphysics guy. Fortunately/unfortunately, Brandeis and NIU are both strongly geared towards epistemology & metaphysics, making either a secure option but together a hard choice. I've already alluded to the fact that I think Hirsch and Fantl would be quite the one-two to lure me into taking a part-time job in addition to student loans just to study with them (and the other faculty); but I'm also afraid Fantl will be returning to Calgary soon (he's only a visiting prof.). One of my LOR writers strongly pushed me to go to UW-M for their epistemology dpt., but it seems more history/early modern; NIU on the other hand, regularly offers pretty amazing courses on the contemporary stuff I'm interested in.
Altogether, NIU is clearly the best choice, right? Well, in a naive-utilitarian, extrinsic-values sense, yeah: I would be walking away with pretty much a free MA and my best chances of getting into Rutgers. So what I'll be thinking through over the next couple weeks is how devoted I would be to slavishly working my ass off to study under the Brandeis faculty for it's own sake. (Also, as is often mentioned, I could take courses at other schools while attending Brandeis; Dennett is of course the popular choice, but I should note that Hintikka seems to be still teaching at BU -- would be pretty cool.)
I'd appreciate any critiques/comments on my thoughts, and I hope if just for the sake of sympathy/empathy/pity, this helps or makes others feel better.
P.S. You said, "It's funny how painless a choice seems when there are no alternatives and how complicated it becomes once an alternative is available."
It's crazy: as soon as I think there's no way I can afford Brandeis (even with loans), my attitude switches to complete indifference; as soon as I think it's just a choice between incurring debt or not, I fear I will existentially regret forgoing Brandeis.
Interested to here where you went, though.