What specific times will you be arriving? If you won't be on the road before 10 a.m., just take the 101 all the way to the City. If you arrive earlier than that, take the 280 North from the southern SJC access road (ask at the rental car counter) and then connect to 101 via the 380 near SFO airport. I've done this countless times and there has only ever been any traffic if there is an accident or I somehow manage to arrive in the City during the afternoon commute over the Bay Bridge (backs up 101N to about the 280 interchange). The 280 North of 380 always seems to have traffic, whereas 101 between SFO and the 280 interchange doesn't ever seem to have any traffic.