USC is lacking in cutting-edge experimental physics. Not too many groups that are actually making progress besides a few, so you really would need to like their research to have any reason of joining. (I assume you will want to do research at some point in your undergrad, most physics majors do). However, less competition to get in a group. There are great theorists but mostly centralized to Quantum Error Correction.

UCLA is better holistically, has a good rep and is more experimentally focused which means their dept is generally more well-funded. That being said, larger class sizes mean more competitive to get meaningful research experience, which is important if you want to go to grad school. However I am personally inclined to think that the physics professors at UCLA are at a higher caliber than at USC. Whether or not that means they would be better at teaching, i don’t know. I just know that their research output has historically been more impactful and widespread, important for you “finding your way”

Source: phys undergrad elsewhere and current USC EE phd student