It's easy to change majors at CMU. People in Pittsburgh seem to be more laid-back than people on the east coast, from what I've gathered, but that's a sweeping generalization. Pittsburgh is way cheaper than Boston. I can't speak to MIT's atmosphere or Boston's social scene, but people here are pretty friendly, and collaboration is however you want it to be. If you never try to collaborate with others, nobody's going to force you to, but there is plenty of teamwork at CMU.
Fewer girls is probably not even true... or at least it's a negligible factor. Don't come here with the mindset that the girls are no good - I made that mistake, and I soon realized that the selection of females is about the same as other places per capita. There are simply very few girls in engineering, so you get fewer attractive girls - it's simple statistics. There are plenty of girls in other majors at CMU, so just get to know people from different majors. I went to a public high school, and I'll just say if you're looking for more... common girls, you might want to check out neighboring University of Pittsburgh or Duquesne. Long story short, girls aren't really a deciding factor here, especially when comparing with MIT.
Pittsburgh isn't really a college town... at all. Pittsburgh is just a rust-belt city with several universities/colleges, so it probably won't come close to Boston in that light. It's a very interesting city with lots going on, but a lot of people never leave campus enough to take advantage of the city. (Shame on them.) Simply because of its size, Boston would have more going on any given night, but Pittsburgh has more than you'd expect for a city this size.
I heard MIT has crazy parties, but that could be urban legend. CMU has parties, but nothing like INSANE. The first weeks of school and the first weeks after winter break are the peak partying seasons (also spring carnival ~April 20). People get bogged down with finals towards the end of each semester. I have more fun with like 15 friends than 200 strangers, so I don't mind the lack of large parties at CMU.
It seems like you know a lot about MIT but not much about CMU... visit? Also, it's pretty nice to live FAR away from your parents sometimes.
Finally, CMU doesn't really have aerospace, so... yeah.