Both UCI and USC seem to be great schools. If the cost differential is big, then that might sway the balance in favor of the less costly option. I interviewed (and was accepted) at USC and I was really impressed with the curriculum, the clinical training options, and the cohesiveness of the student body/support from the administration. The new hospital is state-of-the-art, beautiful, and has a disgustingly large patient volume. I don't know how the fact that it is one of the biggest teaching hospitals in the country affects your education -- it may mean that you will get a lot of hands-on learning, or it could be that there won't be enough mentoring since the higher-ups are all too busy with too many patients to teach you. I asked this during the tour and the two students (both MS4s) assured me that they had amazing experiences during third-year, and that there was enough mentoring time for good learning. The pathology, you may have heard, is incredible at LAC+USC since so many untreated, un-worked up illnesses come through their doors.
If you are interested in research, Keck has a nice summer research option and it looks like they fund anyone who wants to do research at their institution. There also is a competitive year-off program, and they can fund you not only at USC but anywhere else you may want to do research!
You should take a look at medweb.usc.edu if you haven't, it's basically a sample page for the student access site. It has a lot of good specific information on the curriculum, research opps, and administration. You can even look at the different electives/selectives offered.
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