To clarify: it’s not so much that students from UMass would hang out on the other campuses all the time, but 1° they’d get an opportunity, by combining the consortium and the Honors College, to bypass large lectures, and 2° for international students, it means a wealth of free entertainment opportunities.
Commuter schools such as SB rely on what the city offers - unless OP is wealthy, many options in the city will be too costly. And it’s just not the same to “go into the city”. For example, for parties, you need to know someone in the city that’ll invite you. On the other hand, NYC is exciting and a great place to be young (especially if you’ve got money).
100% freshmen at UMass, 84% freshmen at SB live in campus housing… but most of these 84% return home on weekends when only a minority do at UMass. Therefore, local entertainment and community can be found over the weekend in Amherst and on surrounding campuses in a way that isn’t true at Stony Brook. For a “local” student, this isn’t very important, but for an international student who’ll be living 4 years there, it does matter. If OP has friends in the NYC area or already at SB, and if OP has wealthy parents who can fund weekend trips into the city, the situation would be different.
I’m also quite sure a regular undergraduate would not run out of classes at any public flagship and Commonwealth college really does make a difference to the experience, peer quality, class environment, and learning experience. If OP got into SB Honors that’s yet another matter to take into consideration though.
Note that I’m not a UMass booster actually, I’m just used to international students. If the choice were between SB and UMass without the Honors College, the choice would be harder but I just wouldn’t recommend a commuter campus to an international student. OP may well have other choices which would result in his/her choosing neither UMass Honors nor Stony Brook! 
We don’t know whether SB, UMass Honors, UWash, have better graduate placement in astronomy. Most graduate programs would require excellent classes in physics and math. A typical undergrad will take 12 classes in his/her major. OP can look at 1° what classes his/her current preparation would allow him/her to bypass at both SB and UMass, so that s/he can devote more time in more advanced classes; 2° whether there’s a built-in process to take graduate classes if one is advanced enough 3° class size for the required introductory classes that OP can’t bypass and to help develop a relationship with professors 4° research opportunities (ie., not just if they exist, but how easy they are to get, if there’s a guarantee of sorts, what year, etc) and support for undergraduate research (funds for presenting at undergraduate conferences, on-campus conferences) 5° support for REU and fellowship applications (ideally there’d be an office dedicated to that).
Op: it’s really a matter of opinion. Subjective. There’s no “perfect” university. You have to decide what matters to you, taking into account all factors. There are reasons to choose Stony Brook and reasons to choose UMass.
Both choices, and their reason, are absolutely valid.
Based on this
http://www.thecollegesolution.com/the-colleges-where-phds-get-their-start/
Caltech, HarveyMudd, MIT, Reed, UChicago, Carleton, Princeton, Grinnell, Wabash, Lawrence, are tops for PHDs in physics or physical sciences (in terms of ratio undergrad major: PHD). OP, did you apply to any of these?