I have been to NY and San Fran.
Quick Impressions of NY:
- Large, fast, crowded, history, central park, empire state, noisy.
In summary: NY is fun if you like city life. There is lots of cultural activities. It did not feel as unsafe as people claim (in terms of terrorism AND crime like muggings). Eating out is reasonable (compared to the rest of America), but shopping (for food say) is expensive in Manhatten.
Quick Impressions of San Fan:
- California, harbour, trams, hilly, golden gate bridge.
In summary: It's california, so you know the weather is good. You are not too far from LA, which is definately worth a look if you are in CA. Trams make getting around the hilly city much easier.
I don't know if that helped. If you have specific question about either city, ask, and I can try and help you out.