They're both excellent schools. Hopefully you'll get into one of them! They're most certainly top-tier and, even for native L.A. residents, it is really nearly impossible to get accepted and the only surefire bet is if you're rich and powerful and somehow connected to somebody within the school who is also rich and powerful (my observation, anyway).
I applied to both schools, UCLA rejected me and USC accepted me, but with no financial aid at all (leaving me with like, thirty-eight grand I had to shoulder annually...). So I ended up at neither. But I toured extensively at both schools and found I enjoyed UCLA's brighter-feeling, more bubbly campus to USC's dark, brooding, gothic fortress-like atmosphere (purely aesthetic, I know, but this stuff was important to me!). Plus USC is in a scary part of L.A. and I felt pretty safe at UCLA.
Academically, I think you'll get a fine education at both but my preference is more towards USC because of the high caliber of former as well as working professionals on staff versus UCLA, where I met a lot of knowledgeable professors for sure, but didn't find a lot of... err... Hollywood bigshots, I guess. Hey, that's why schools like USC use their money to lure pro's in to teach - it IS a selling point to potential applicants. But again, they're both great schools as far as I could tell. And the "getting in" part isn't exactly a no-brainer, so apply first and see who takes you!
You might also want to consider a number of other Los Angeles film schools for a film music program. UCLA and USC are big names but that doesn't mean any other school is subpar. I can't think of any others off the top of my head, but Google ought to help you out there (my own school is attempting to get a film scoring program going, but it's not quite there, yet).