UCB undergrad here, who visited friends in UCSC & UCD fairly regularly.
I am a city person who likes to stay up late and go out to do things–not bar scenes or parties, but movies, live music, museums, theater, dance, meetings, etc. There was never, ever a lack of things to do in and around Berkeley (including SF)–none of which you needed a car to get to.
Davis drove me nuts. I found it boring–less interesting in the diversity of people and fewer things to do. YMMV so if outdoorsy stuff is OK with you (and you’re OK with few other alternatives if you get tired of the outdoorsy thing), than Davis would be fine.
As for the geniuses, sure there are a few, but they aren’t the ones you worry about. Most people going in are in the same boat as you–confident in their abilities but maybe a little insecure knowing they’re competing with a lot more people equal to you. It all comes down to studying and applying yourself. I can’t attest that Davis is less stressful, especially for Biology; it seems everyone who wants to go into Ag goes to Davis, so the competition in Bio will still be pretty fierce.
Transferring to Davis from Berkeley wouldn’t be a problem if you found you didn’t like UCB (but the reverse would not be as easy). The semester vs. quarter thing is also pretty big. All my friends at the other UCs never seemed to have long enough breaks or weeks that didn’t involve a paper or midterm. Semesters allow you more time to settle into a groove–less stressful, it would seem (though everything does count for a little more, I guess).
I wouldn’t worry too much about the rep.
Worse. Unquestionably. You want to situate yourself in an environment where you can excel. I found this meant having lots of options if I needed to relax or unwind, and knowing that I was getting a lot out of my life personally (socially, politically, professionally, whatever) outside of academics. Berkeley certainly fit the bill for me (much more than Davis ever possibly could have), but I’m the first to admit that Cal and the surrounding city is an acquired taste and most definitely not for everyone.
Good luck in your decision, whatever it is.