Originally, the UC's were the only institutions allowed to grant doctorates. That hasn't been the case since the 1970's, but only for several select majors (like Education). The UC charter requires offers of admission to all qualified students who are in the top 12 1/2 % statewide, and who complete all required classes and admissions tests. The CSU's take the top 25%. There is a lot of overlap in these groups, and the more established CSU's like San Diego and Cal Poly SLO offer a cheaper but just as good alternative to some of the less established UC's. The CSU's have a lot of proud alumni, and their graduates get a lot of respect here in California. Where I live in San Diego, nearly everybody on the County Board of Supervisors, in Congress and in City Hall is an SDSU graduate. UC San Diego doesn't even come close. Also, there are big time sports programs at schools like Fresno State and San Diego State, which aren't offered at the nearby UC's, so that is an added attraction to students.
Go where you think you will fit in best and be happiest.