For me I'm driven by fares and schedules. If those two were comparable I would always fly out of SFO.

OAK used to be a great quiet alternative to SFO about 10 years ago. Not any more. It is so rundown compared to then that it is quite depressing to spend any significant amount of time there. In fact, if I have any serious delays out of there I look into trying to depart SFO, even if it means a longer overall trip and the need to get across the bay.

Nowadays I use OAK when I'm forced to ride WN or when I can get a nice discounted F ticket to and from the east coast which are hard to come by out of SFO.