If tuition wasn't a factor, I would definately choose USC.
The clinical training at USC beats the S**t out of almost any school. At county you get to see the things you only get to read about at other schools.
As far as living environment is concerned, I think USC wins hands down. The majority of the students at USC live in a place called Monneray Hills, which is just south of South Pasadena. It is about 3 miles away from USC, and it is in a way, isolated. There are only three roads into monnerray hills, and basically on the hill are a bunch of very nice apartments and suburban like houses. Also you are 10-15 minutes from tons of entertainment in Hollywood and downtown LA, and 5 minutes from Pasadena, where there is lots of residential stuff. UCI is in a very nice place, but also extremely boring. There is not much to do in orange county. The clubs arn't that good (except rubber) the bars arn't that great, no real sporting teams. There is the beach, but LA has that too. So for me, LA would beat Irvine for living.
One thing that I think UCI has USC beat in is the basic science training. UCI aparently scores very high on the boards. I'm not totally sure why that is. It could be because for the first two years they are mostly in the classroom compared to USC, where you in the hospital once a week your first two years. Could be that UCI selects studnets with higher MCAT's and GPA's than USC (my personal opinion).
One great thing about both schools, it's extremely easy to get a residency in California. You can't really go wrong with both.
So if I were you, my deciding factor would be how well you think you can do on the boards. If you did well on the MCAT, you probably won't need to much help on the boards, so I would go to SC because the clinical is much better. If you feel you would be in the middle of the pack on the boards, then you should go to UCI because they will prolly teach you better and youd score better.