Hey, so I got into both UCLA and UCB and chose Irvine over both. This was primarily because I thought that the opportunities I'd have at Irvine would be most available to me since I'd be more of a "big fish". I furthermore thought that given the same amount of "working hard", I'd get a lot more return at Irvine that I would at UCB or LA. I think I've found this to be true (as much as one can tell) once I attended and have not regretted my decision since. You should know, however, that money did play a role and the scholarship I was offered was $9000/year and $1500 over summers. The scholarship currently being offered is considerably less, though it increases dramatically with need-based aid. The Campuswide Honors Program (CHP) is a whole other part to why it was worth it to go to Irvine, so I would definately immerse yourself in it if you were offered membership in its program.
Now. That being said I do wish to bring up the fact that while traveling on interviews, people from UCR, UCI, UCD, UCSB, UC..Merced, UCSC were notibly scant. State schools from Cali were virtually absent. UCSD, UCLA, and UCB were HUGELY dominant in the people being interviewed (in the 9 I went to). On the other hand, all of my friends who are also in the CHP at Irvine currently hold acceptances to medical school. I think they told us that 95% of applicants from the CHP with a 3.7 and 30 MCAT or above got in somewhere. Furthermore, I have friends who plan on going to UCLA, Wash U, and I most likely will end up at Northwestern, so it's not like going to Irvine kills any hope of going to a good school.
Overall you will just have to weigh your priorities and where you think you'll perform the best. If you can pull off a 3.85+ at Irvine and a decent MCAT, you'll do fine. If you think you'd have a better chance fighting with the masses at LA or Berk, go for it.
Hope that helps, let me know if you want clarification.