i currently live in LA and grew up here, and also lived in the SF area for 5 years (left last year).
i suppose it depends what you are into, but tommy2000GT is right. SF really is just some sight seeing and nice views and scenery and nice restaurants. i'd say more eclectic neighborhoods and more hole in the wall bars with "character". LA has a lot more glitzy nightlife and has better weather. bay area i guess the topography and such lends itself to really nice views and hikes and places to take pictures from. like berkeley hills is even a nice place to just kind of drive around.
i'd say if you are going to SF for the first time, you should take cabs or the trolley around the city and see weird neighborhoods like haight and ashbury, that windy street , walk around north beach / marina , maybe take the ferry , wander around union square for like a few minutes, and if you are into it see the mission district if you want to see hipster town or smell heroin addicts. i guess theres sonoma and wine country. and more "naturey" type stuff though i never got into that while i lived there.
seeing as im a programmer a lot of my friends lived in silicon valley and i'd have to say if you werevisiting, there is almost 0 worth going out of your way for down there unless you really wanted to see the stanford campus or like 50000 office buildings and suburban grid style neighborhoods (i.e. like all of sunnyvale and cupertino). half moon bay is an ok beach, but the ones in LA are great beaches. monterey i've never been to , but i've heard its pretty nice, and you might want to stop by santa cruz which is a pretty cool spot, the boardwalk is decent. oakland has jack london square and downtown oakland is allright as well but i can't think of anything i'd say is a must see.
again it depends what you want to see. if you and your gf want to see ridiculous night life, when you are in LA, you can hit up downtown LA, hollywood, etc. theres a pretty nice bar scene on the west side in say santa monica . venice beach is nice during the day. if you are into clubs, the electronic dance scene in LA is about a 1000 times bigger than in SF. there are tons of overcrowded hip hop clubs. theres the usual toursit spots like grummans theater etc.