I've lived and worked at startups in both NYC and SF. It isn't even a contest. The amount of startup activity, the character of the companies (ie are the foun

I've lived and worked at startups in both NYC and SF. It isn't even a contest. The amount of startup activity, the character of the companies (ie are the founders left in charge or do you have to have a Harvard MBA to pass muster with the VCs), the ecosystem surrounding the companies, cheaper housing, the ability to move between companies (you mostly can't enforce non-competes in CA) -- come to SF. Oh, and the weather. Paul Graham will give you the same answer in a much more detailed form -- go read it.