Very different atmospheres. IIRC, McGill is an intensely urban university, more like NYU or perhaps Georgetown in comparison to the other two. Montreal, of course, is an easy place to have fun, if you have the money to go out every night.
I’ve never been to Rochester, so I can’t comment on how fun it or the town is. I’m sure that academically it has much to offer in the social sciences. Poli Sci and pre-law are not exotic subjects.
I’m a Wesleyan alum and can attest to the fact that it can be a fun place. The party scene begins freshman year with at least one dormitory becoming widely known as “party central” (it shifts from one year to the next., so I can’t tell you which one to apply to - or, avoid for that matter.) And, the campus is surrounded by hundreds of one-family houses that have been converted to use as dormitories. Each one has its own kitchen and “house parties” have become a Wesleyan specialty. Also, if you’re a sports fan, the football field sits right in front of the main library where you can follow the score from the main reading room (in the spring, it doubles as a baseball diamond.)