I applied to both schools (and got into UCD, still waiting on UCSB), and to be honest I don’t think the difference is large enough to concern yourself about. If I really hated the party atmosphere, and every other factor was equal, my personal preference would be UCD since the programs are so similar in quality.
I’d love to hear from someone who has attended UCSB though. The partying may or may not be something you can distance yourself from.