I didn't get into Harvard, but I chose Boalt over some of those other East Coast schools (Columbia et al.) for a lot of the reasons you mentioned (family, friends, significant other all in the Bay Area, already lived here and liked it, etc.), so that's where I'm coming from. The opportunities at Harvard certainly outweigh those at Boalt, perhaps significantly if you're shooting for ultra-competitive things. No question about that. That said, it depends on what you want out of your career. You will have stellar opportunities coming out of both, especially if you figure out what you want and push to make it happen.
I think you are definitely over-estimating the extent to which Harvard is full of robotic Type-A stress-cases. Every law school has some people like that, and both of these law schools will have plenty of people who are more laid back. If I were you I would be focusing more on whether you want to pick up your life and move to the East Coast for three years v. the greater career opportunities that Harvard would bring you, rather than the vibe of the student bodies.* I have found that being around my support network during law school has been invaluable, and I think it's made me do better academically, and manage my stress better. But Harvard is Harvard. I guess I'm saying I don't think you'd be crazy to choose Boalt, but I don't think it should be because you worry about your classmates at HLS being lame. Good luck with your decision!
*More of a tangent that is not really useful to the OP. I kind of disagree with this folk wisdom that there is a perfect correlation between more opportunities and less stress. While that is certainly true on a macro-level (particularly when comparing schools where a large number of people really have to worry about not getting jobs v. schools where most people pretty much get a job), I don't really think it's so clear when comparing, say, UVA and U Chicago, for example. Some schools do have distinctive cultures. Really they do. My friends at Stanford are just as competitive and stressed-out (if not more so, honestly) as my classmates at Boalt. They may have more opportunities, but they are just competing for the things one notch higher on the totem pole. There is always something to compete for.