Im a married 35 yr old professional moving to San Fran, whats the best neighbourhood to live in?

I work in London for a large US financial institution and am close to agreeing to move to San Fran, subject to agreeing terms. We're really concerned about the cost of rent in SF but have no idea which area to live in. Some friends tell me that 'we must live in the centre of SF to really enjoy the experience' others tell us to watch out because there are lots of areas that seem safe but have streets/areas to watch out for. Any suggestions of areas for us to consider would be really appreciated. My wife will sadly need to quit her job and will need to wait for a visa before looking for work. There's also no guarantee that she will find work as her profession doesn't translate too well in SF (in the UK she is an Deputy Company Secretary). Therefore my salary may need to cover us for the forseable future. We may also thinking about starting a family in the next 9 to 18 months which might be important for us to consider away well with out rental location.