Hi Sky Pilot, as a current M.Div. candidate at Andover Newton graduating this May, and as someone who was facing a similar decision between Andover Newton and Harvard three years ago as you are now (was accepted both places), let me throw a few thoughts into the kettle:
1) While certainly Harvard has lots of amazing interreligious programming, Andover Newton is no slouch in this department. We share a campus with Hebrew College, a rabbinical school, and that proximity is utilized with a strong relationship between the institutions, including interfaith classes offered jointly between the two schools, an interfaith organization that has a half-time director (who is also an associate professor of interfaith studies at Andover Newton), and much interfaith programming. Not the same interfaith environment as Harvard by any means, but not lacking in that area either.
2) I am also Unitarian Universalist, and I have really enjoyed my experience at Andover Newton. We're not perfect in supporting UUs as a Christian seminary, but we're pretty good, and the support from UUCANS, the UU fellowship group on campus, is phenomenal. UUCANS is the most active fellowship group on the Andover Newton campus, and they/we offer lots of great programming throughout the year, much of is offered to and attended by other UU students in the BTI, including UUs from Harvard.
3) You mention academic rigor, as does the person who responded to you. While I am not an expert on Harvard classes, and while I would guess that Harvard classes are indeed more rigorous than Andover Newton's (based on the handful of classes I sat in on while in my decisions-making process), it has been far from my experience that our classes are not rigorous. Just because they might not be as rigorous as Harvard's doesn't mean that aren't rigorous. I have attended some elite institutions during my academic career, and Andover Newton is no slouch. I have felt plenty challenged and learned a great deal here, no complaints from me.
4) In terms of your respondent, Andover Newton, is of course more geared towards producing practicing ministers than Harvard is. It depends, I think, on what you're looking for. If I was planning on being a professor and pursuing a PhD, I would have picked Harvard hands down. I was and am much more interested in ministry and the art and practicalities of ministry, however, and so Andover Newton was and is a much better fit for me.
5) Though your user-profile says you're looking at the M.Div. program, it doesn't say whether you're looking to be a minister (as I am). I have served on a search committee in a UU congregation, and I can tell you the place where the candidates went to seminary was barely discussed at all in our group. Having done some brief informal surveys of other lay leaders about this, I can report back that in search committees where it did get mentioned, it was a very small part of the discussion. Folks are much more focused on your words (the UUA requires a lengthy set of responses as part of the application process) and your experience, than where you went to school. So practically speaking, it's cool to get to say you went to Harvard, but probably doesn't have as big an impact in the world of getting a job as a minister (or at the very least a UU minister) as it might in other professions. If you're looking to do something else with your M.Div., the Harvard name is much more likely to make a difference.
You say "money aside, Harvard is my first choice." I get that. I ended up spending a lot of time agonizing between Harvard and Andover Newton, but only because I faced a similar situation as you with my first choice (which was Union Theological in NY). I loved many things about Union, but the money from them was way less than at the other two, and I just couldn't justify to myself going an extra $45k into debt to go to my first choice, when I had two other excellent options available (Harvard and Andover Newton) for a much lower cost.
I don't know that I have any advice per se for you, except to you tell you that I went with my "second choice" that cost me a lot less, and I have loved it and had an amazing experience. Is it the same experience I would have had at Union? Absolutely not. But I can tell you I honestly don't regret it -- my time at Andover Newton has been really great. And I am very glad not to have that additional debt. That was my choice though, and not necessarily the "right one" for you or anyone else.
One other "decision-making" thought -- the advice your first respondent gives about the BTI could also cut the other way. If you're worried about cost, you could go to Andover Newton and take a whole bunch of classes at Harvard. You wouldn't get the name on the degree, but you could get a fair chunk of the experience.
In terms of funding, if you're in the M.Div. program and planning to enter the UUA's ordination process, you should do that ASAP. While they aren't huge scholarships, they do offer yearly financial aid to folks in the ordination process who are in seminary, and every little bit helps.
Regardless of where you end up, you will get an excellent education, so in that sense at least you're in a win-win situation. Good luck with your decision-making, I know how tough it can be!
-Soon-To-Be-Andover-Newton-Alum