I've lived in both places - public transit in SF beats the pathetic pants off of Seattle for sure. It won't hold a candle to NYC though, so you may be disappointed.
It's entirely possible to live without a car - very, very much so, but be prepared to pay if that's the case (i.e. neighborhoods well served by transit are expensive, think Manhattan rents... up to $2K for a studio? You bet!).
It's entirely possible to live without a car - very, very much so, but be prepared to pay if that's the case (i.e. neighborhoods well served by transit are expensive, think Manhattan rents... up to $2K for a studio? You bet!).