I hate it when I don't get replies, so I'll give it a shot. My info all comes from a categorical view, and I don't know how it would differ for prelim.
One of the pretentious attendings with whom I've worked said that this was the best training in LA, second in cali to UCSF- I am not sure why/how it beats UCLA. But according to him, and other sources, the training there is good.
I met someone on the trail who interviewed at USC and he actually really liked it and liked the interview (and I talked to him alone to get the dirt)-he encouraged me to interview there for categorical. He ssemed to be highly considering it, and he seemed to be a happy outgoing guy.
For categorical they have an otpional day before the interviw that is really required, and I couldn't make it, and was less enthused about the area (really $$ or really violent) so I decided to cancel.
They do a lot of county work, so high autonomy, low funds, can be good or bad, depending on what you want and need.
UCLA has a few residents who transferred over after intern year at USC becuase it was bad for them, I think in terms of culture and morale. They wouldn't talk about it, but there eyes got weird when you asked why they moved over.
I know this might not be what you were looking for, but I hope that it puts some light on your decision!