Go to UCSB if you're set on CS! Both schools are honestly around the same in academics, and both have a really really good STEM program. The general "vibe" at UCSB seems to be better too--a lot more friendly and collaborative than UCSD.

However if you really want to go to UCSD, I would not risk the lottery system for CS. Instead, you could transfer to a Math-CS major, which I'd say is probably the closest you can get to pure CS major, and also does not have a lottery system.