They're really really different. I was only at UCLA for summer quarter before I went to UCSD for this Fall quarter, so I haven't been at either that long.
They're very different culturally. UCLA feels like it has a lot more going on than UCSD. There are constantly events on campus: movie screenings, live bands, sports, etc that you can't miss so it feels like there's more going on. That's not to say that at UCSD there aren't things to do here, there are events practically every day at UCSD, you just have to look a little harder. Culturally, I prefer UCSD; I like the laid back atmosphere. I like that it's actually quiet on campus in November and that I can walk from Muir to the beach in 15 minutes and go surf (Black's beach/Scripps Beach > Santa Monica/Venice). UCSD is about 5 times larger than UCLA with about 6000 less students, so people are more spread out and it tends to be less hectic. Although around the central areas: Geisel Library, Price Center, Library Walk, etc during lunch and between classes it's as busy as any other campus.
The one thing I really miss about UCLA though is the school spirit and sports. I'm a huge college football/basketball fan so not having any D1 teams here to go watch is pretty disappointing because that's something for me that I've always associated with the college experience. That said though, La Jolla compared to Westwood is a no brainer in my eyes. La Jolla is SO much nicer. Close to the beach, no insane traffic, and I live in a really nice apartment off campus, that if it were in Westwood, I know I wouldn't be able to afford. IMO, housing on and off campus is better at UCSD than UCLA.
Regardless, the reason why I enrolled here was not because I didn't like UCLA it was actually because I got accepted into a fellowship program here at UCSD during the summer that pays about a third of my tuition along with a $1000/month stipend to work in a research lab. It was an opportunity I couldn't pass up. Currently, I'm working in a lab that investigates the proliferation of certain cancers and how tumors metastasize and attack blood vessels. I'm a Molecular Biology major and I'm so thankful I ended up here because UCSD's biology department is so influential, it's the biggest department on campus, and with that comes so many opportunities and resources that I think even surpasses the reputable life sciences department at UCLA. That and there are so many research opportunities on and off campus that it's a great environment for someone that wants to get into a research lab. There's not only UCSD, but a multitude of research facilities associated with or located near the school like UCSD School of Medicine, Sanford-Burnham, Scripps Institute, Sanford Consortium, etc that it's a really great place to be for someone interested in biology or medicine as a career. UCLA has a lot of great research opportunities as well, I know a big stat they throw around is how they get close to $1 billion/year in research funding. That said I think for someone with my major UCSD is a great place to be. UCLA is more rounded academically, so for someone studying in the humanities or social sciences I'm sure the opportunities at UCLA are more appealing.
When I started typing this I didn't intend to write so much, but inevitably it happened. I could really go on and on about the two, but I'll cut it short after that lengthy page of text. If you have any other questions or if there's anything specifically you want to know just PM me.