Wow, what a lovely dilemma UMass v CMU. I’m interested in what kind of student you are, how strong. Also what you envision, will you want to work right away or maybe get MS first then work. Or are have you been thinking over a PhD at all? I heard from my own daughter of the luxury of engagement in class and research with your profs at a smaller private school, compared to what she observed as a grad student TA at one of the publics on your list. Not engineering but CS. However the cost of UMass instate is a massive good deal for still a strong engineering school.
Interesting what a student does when it is now their own money on the line. I do know two siblings who had the choice you do and one went to the free engineering school and banked the money and is using it for med school. The other went to the expensive school in humanities and now working at google, lol. Also my relative did EE at Princeton and did football the first year but had to drop it, it just wasn’t feasible.