The two schools are very different in many ways. I suggest you visit both and get a feel for them. When it comes time to apply to medical school, your grades, MCAT, and ECs will be what determines if you get in or not. Whether you went to UCB or UCSD really won't matter at that point. I went to UCSD, I can tell you for a fact there are tons of great classes, research opportunities (if you don't take advantage of this you would be foolish), programs that will help you shadow/get mentorship with docs (this is mostly for juniors & seniors), and volunteer experiences. In fact, the campus includes three hospitals (VA, UCSD Thornton, & Scripps Memorial), has a shuttle to a fourth (UCSD Hillcrest), and has a fifth up the street (Scripps Green). Also, the area surrounding the school, besides the beach, is mostly biomedical research companies (internships are available through the school if you are interested, although, they are probably more for if you want to work in a lab eventually instead of med school).
I would not select UCSD b/c you think it is less competitive. The level of competition is tough, although, I did not find it "cut-throat" or even unfriendly. It is what you would expect at a school full of bright and motivated people (although the usual D's and F's are present in every class). Most pre-med classes have 300+ people, some classes curve, and some classes don't curve at all. 300+ people is a lot, but it usually results in a fair distribution of grades, and we have weekly "discussion sections" with TAs that consist of ~20 students.
UCSD's campus is beautiful, quiet, has lots of places to eat, several libraries, lots of study lounges, and free campus wide wifi that works well. I'm sure Berkeley has all these things too... if you can look past the bums and hippees that you share the campus with... I kid, I kid... but it's true 😛. One environment, UCSD, is mostly suburban (downtown San Diego, pacific beach, downtown la jolla, the closest beach are all <15 minutes driving). The other, UCB, is very urban immediately as you step off campus. I think you need to decide what is more your speed, and what you like. I lived in San Francisco for a year, and I learned that I'd rather visit than live there.
If it were me, I'd also consider how far each school was from my friends and family.
Good luck, I think you have two great options and you can't go wrong as far as the pre-med stuff goes.