Z might be bigger, but are the CS class sizes larger? I went to a fairly large university but it had a small CS department and most classes had <25 students which meant you got a lot of attention from your professors. Also keep in mind that CS curricula are more or less standardized by ACM/ABET so the largest differences will be in electives/research projects.
Arguably the most important part is social. You've read SICP and already contribute to OSS so I'll assume you already know you can teach yourself. Understand that modern university is mostly a social thing and less of an educational thing. Look at the clubs on campus and see if you think you'll enjoy them. Most of my friends and a couple people I dated came from the robotics club for example.
Don't underestimate the value of the financial aid. Not having everything paid for can add a lot of stress and once you start getting internships you'll probably want to use the money for side projects/investments/dates etc.