I went to SC, and a friend went to UCLA. As others have said, if you're a motivated student, both schools will provide you with the resources to succeed. With that being said, my friend often complained about the hundreds of students in most of his classes as well as a heightened sense of competition. SC also has a lot of motivated students, but I felt that with the generally smaller classes it was a lot easier to talk with the profs, network for research opportunities, etc. I never visited my profs during office hours unless I was required to, but each of the ones I emailed for LOR's during my second gap year remembered me and were happy to write.
Although I have to admit UCLA's location in Westwood is much nicer than SC's in South Central, as I'm sure you know. But the Village has been a nice addition during the past few years.