There are ample opportunities for bright students to excel, find their people, and engage in intimate communities at larger schools. UMass in particular provides plenty of access to faculty, TAs, grad students and is a warm and pretty laid back atmosphere. Some may prefer LACs for various reasons but it’s a fallacy that smaller student bodies or class sizes necessarily yield better or deeper dialogue or connections. I’d go with UMass if that’s what feels like a good fit, and I wouldn’t treat it as the lesser of those two choices, just different. Whether Cornell would be better and worth the extra cost depends on fit. Very strong in the sciences, obviously, but probably has more stressed-out students than the other two (much of it self-imposed, but that does influence the culture).