My rule of thumb is to live either right where you work or right where you play.
Living in between means everything has a little bit of hassle attached.
San Francisco: notoriously insufficient sun, plenty of beaches (though the water is unreasonably cold, and they’re often foggy), giant happy park and other good parks, the most fun of your choices, plenty of cool companies.
Not a short commute to/from Mountain View (depends on your desire to commute down for work or commute up for playing in SF); also it’s priced accordingly (very high).
San Jose: 10-20 degrees warmer because it’s far, far sunnier, nearest beaches are Santa Cruz or Half Moon Bay which are sunnier but still cold water (and get trafficky to get there on the weekends during the summer); larger companies, not as entrepreneurial, more suburban entertainments, and Cupertino is particularly family-oriented. “The Peninsula” (Mountain View to Redwood City) is suburban, pockets of wealth, more boutique-y entertainment and the fun and cool companies would be near Stanford (Palo Alto).
Nearest beach is Half Moon Bay or Santa Cruz.
Agreed you’re much better off if you can visit first.
If you can’t take a trip out to look around, I might suggest parking yourself in a sublet in Mountain View itself while you get oriented – or even breaking my normal “in between” rule, Palo Alto is reasonably fun, and you should be able to find a sublet during the summer b/c it’s a college town.
You might find you want to live in Santa Cruz and commute (it’s a bad one if you do it during rush hour, but you could commute odd hours and live at the beach), who knows?
More Resources on If my new job is in Mountain View California and I really like the sun and the beach is it better to live in San Francisco or live closer like San Jose or Cupertino:
Read more about If my new job is in Mountain View California and I really like the sun and the beach is it better to live in San Francisco or live closer like San Jose or Cupertino: