Either way, depending on the student.
A premed with a poor chance of admission may prefer the lack of any committee letter at UCLA if they want apply anyway.
But one who wants to know if it is futile to apply early in order to stop wasting time and money on premed in order to find some other direction may prefer the committee letter prescreening at USC.
Probably most frosh premeds would prefer the UCLA method, but probably most rejected medical school applicants would prefer the USC method in retrospect.