SFO is closer to downtown SF and also has a direct BART connection. If you take BART to OAK, you'll have to get off at the Coliseum stop and then purchase another bus ticket that will take you to the airport. Cab fare will be much less to SFO than to OAK.

IMO, SFO is a much better airport. OAK is pretty small and not very nice. It is also not a very business traveler-oriented airport, probably because it has the likes of Southwest and Jetblue. I definitely prefer SFO to OAK.