If this student is serious about postgraduate study, which he seems to be, it behooves him to accept the program with the best graduate placement in his proposed field. It also seems that he pretty much understands this.
I’m not knocking the honors program at UMass. But from my experience, I don’t see how this would propel him forward as much in his proposed career, as will succeeding in the strongest astronomy (and possibly physics) programs he has on offer. I think it is a bit of a disservice to make the consortium sound like these other beautiful colleges – Amherst, Hampshire, etc. – will be his homes away from home. As ucbalumnus pointed out, it’s more of an advantage for the colleges to fill academic gaps at big UMass, plus maybe an occasional social function or friendship between campuses. I had close friends attend Amherst College, Mt. Holyoke, Hampshire, etc., and I never had the impression that UMass students were there much at all. I had more of an impression that these individual colleges have a fairly exclusive campus feel, but YMMV.
Yes, Stony Brook has some commuters. Part of the reason it does is due to cost savings – it’s a pretty dense area, and not far from NYC. But there are plenty of residential dorms, activities and the like. Last I visited, there also seemed to be a fairly diverse student population, with many nationalities. I took this as a positive, and possibly the NY metro area also contributed. UMass and the colleges are situated in a more rural area, and are thus primarily residential.
If there was an even better option than Stony Brook for him, I’d recommend that (and it’s not clear UW-Seattle necessarily is, as Stony Brook’s smaller size may give him better access to faculty and facilities, and UW is more expensive). I don’t mean any disrespect, MYOS; I just don’t see the honors enticement for UMass as tipping the scales for a future astronomer.