It's been a few months since you moved down into the Valley from the City. Can you give us a stream of consciousness on the topic? :-)
Yes. Stream of consciousness... commence:
First off, I am not living in an a 'normal' way in Mountain View, as others here are probably living. I am living in my startup office. Yes, in my office. Won't go into too much details on that because it's not the point of this answer. But you can imagine, my perspective of the Valley is of a fairly lean-living founder. So take that into mind.
Mountain View reminds me of home, I was born and raised in Pasadena, CA. So it's very familiar and incredibly safe feeling. But I don't get any of the interaction / spontaneous meetups with friends and new acquaintances that I'd get in SF. That is by far the biggest drawback for me as a founder, and just as a young guy in the Bay Area. More isolated from friends, more focused on work and taking it easy when I'm not working (instead of hanging with friends and staying up really late having fun).
Food, good weather, and tech people are abundant in Mountain View; but food is driving/biking distance (not walking), and tech people are farther apart so less mingling in the streets/housings as you'd see in SF.
San Francisco is probably more fitting for me at this point in my life. There are tons of young people, a majority of my friends live there, food is always walking distance in downtown (where I lived), and SF is perfect for long-boarding ;)
But as a startup team, Mountain View is a better fit for us, since most of us live around here and it's quieter, more focused, safer, and pleasant weather / smells outdoors (unlike SF). It's harder to hire people in Mountain View though. If we were to hire someone, they'd most likely live in SF and have to commute or work remotely (which is not preferable for either of us). SF _does_ give me that "city energy" feeling. And it's possible to start a meetup out of thin air in SF as compared to Mountain View, which has considerably less free/young people and less venues.
Final thoughts. If I was alone working on my own startup, I'd choose SF over Mountain View any day. But if the team grows and people need to drive to the office, and eat out together, and feel safe, and feel those 'sunny weather vibes,' then I'd choose Mountain View - where we are now.
TL;DRFor me...San Francisco = energy, small team, meeting peopleMountain View = comfortable, larger team, focus