San Francisco's a cool place. Literally cool as well! lol GREAT nightlife there and if your only there for maybe a week, plenty to see during the day's. Don't know if I'd bother with the Tenderloin area (unless you like TV street walkers? Maybe its your thing).
I prefer San Diego for the weather, beaches (Do you agree La Jolla is the best beach in California Sara?), you can go across to Tijuana for a wild night, the town itself is pretty good and there's plenty of bars around.