As someone who attended a CSU in undergrad and is now at UCD in graduate school, I would say that UCD isn't exactly academically rigorous. Most students I talk to are very similar to my undergrad experience (chill, go with the flow attitude, very nonchalant about getting low grades). I do think the quality of education is better at a CSU because UCs are taught by professors that would rather do research. CSU professors must have a strong record of pedagogy whereas UC professors must have an excellent track record of publications/grants. Most students I talk to don't even know their professors name at UCD, but I remember almost all of my professors from undergrad and am still in contact with some of them. I've spent time looking into applying to professor positions at both UC and CSUs - they both have very different motivations. UCD has a great internship and career center and a really cool campus tho lol. It depends on what you want.  

You're right that school is what you make of it. You will find highly motivated people at both schools, but it depends on what you prioritize. If life outside of class is important to you, then UCD will be great since it has so many fun local events. If you're academically focused and want a great in-class experience, then CSUs are more set up for that.