I was sort of in this position last year.
For the record, I do know of a few people who ended up choosing UCD over UCSD for med school. However, those people had also gone to UCD for undergrad, and I think they just felt immensely comfortable in Davis, they already had places set up to live, friends, in the area, etc., so I'm sure that helped.
As for me, in the early winter I was deciding between UCSD, UCD and Tulane. I picked Tulane over UCD because I just couldn't see myself being happy in Davis for 4 years -- it's a cute college town, and close to skiing, but beyond that, I think I would have felt sort of stifled in the Davis/Sacramento area. I just needed a break from small-town life, and I also think you get more out of going to med school in a big city -- you just see a lot more.
Tulane vs. UCSD was tough. Ultimately I went with UCSD, largely in part of cost and location, but it was an agonizing choice. Mainly it came down to cost -- I just tried to imagine how I would feel with $200+K of debt in 4 years, and that sort of made the decision for me.
I didn't even apply to UCI just because I had heard lots of negative things about it. So, I can't really comment on it personally.
I think you also have to consider what you might be interested in doing with your career. If you think you want to go for a very competitive residency, subspecialize, go into academic medicine, etc., you would probably be better off at UCSD because of their amazing research resources. If you are definitely interested in primary care (family practice, peds, ob/gyn, etc.), UCD is EXCELLENT and you can't go wrong. Tulane is sort of in between -- it's known as being extremely clinically sound, but it definitely is lacking in research resources, especially compared to a school like UCSD.