First off, congrats on both admissions!
I went to UCSB for undergrad, really loved it. Beautiful campus, awesome faculty, amazing CS program (and it boasts the most Nobel Prize winner of all the UCs). In terms of diversity, I'd give it to UCSB. It might not be an important factor for everyone, but I appreciate a mix of folks. The USC program I applied to (and got rejected from) during this admissions cycle was 85% white.
Anyway, you're golden either way. Just had to put in a good word for the Gauchos!
Good luck!