I am still not too sure about the role of the CSU system at the graduate level. But it doesn’t seem as obvious as going to a JC and then transferring to a UC at the undergrad level for those who don’t make it in straight. If this was the case, it would seem obvious to do the Master’s at SDSU and apply to UCSD for PhD. But you are competing with people all over the country and the world and I have no idea if SDSU has a good record of sending their kids to UCSD. Maybe you should ask them.

If you do choose to earn a Master’s degree somewhere first, whether it be a CSU, private, or a out of state, you should find out their placement rate into PhD the programs. It seems like some programs, especially at CSU accept everyone who wants to learn.

You should contact UCSD and find out if they prefer their applicants to earn a Master’s first because for some departments such as CS and EE, prefer it. But I think the best people to get advice are from faculty members at your school because they have been through it.

I have some stats for you at the link below such as acceptance rate, average GPA, and GRE scores.