If taking public transit, namely BART, it's easier to reach SFO than OAK. You can take BART to the Oakland Coliseum/Airport stop, but then you have to get on an AC Transit bus or a "BART shuttle";; BART takes you directly into SFO's International Terminal, no change of system necessary.
From Downtown SF it depends on what time of day but at OAK it's really easy to get through bag check and security on your way to the gates. SFO is faster using BART but the security and bag check and gates seem further. Flights out of OAK are sometimes less expensive. Really its just a personal preference that can only be determined by trying them out. Have fun and fly safely!