You might want to note that MHC is usually about 25% international students. This is relevant both for political science (their Model UN is… kinda the UN) and diversity.
The bus is free. From MHC it’s easier to get to Hampshire, Amherst, and UMass than it is to get to Smith. Cross-enrollment depends a lot on the subject. My Smithie never got off the waitlist at MHC for a poli sci class, but there were numerous MHC students in her architecture studios at Smith. She was able to cross-enroll at UMass without a struggle, just a bunch of paperwork. Regardless, if you make the effort, you don’t have to be isolated at MHC. Participation in events across the consortium is pretty easy.
People do transfer between the schools from time to time, but usually it isn’t necessary because there is plenty of interaction.
Good luck with your decision!