[MENTION=20561]mstraveler[/MENTION],
Do stick with the midsize. The SF downtown locations will upgrade you to a Full Size (Standard is practically non-existant there, if they get an occasional Mustang, they will rent it to someone paying for it.

As for getting to SFO, Bart train is the most efficient (and cheap). Also you could try a one-way from the airport - sometimes they run deals in SF as too many cars are returned to SFO

If you have time to spare and are really curious about the choices, you can go to the last floor of the Mason Street location (it's a public parking garage) and see what Hertz has parked there