In Civilization IV, both workers and settlers prevent your city's food store from increasing for as long as they are being built. Surplus food is funneled into the production of both units, speeding their creation.
A common dilemma I have at the start of the game is deciding which one to build first. I'd like to get a settler out as quickly as possible, to start another city. But without any farms, surplus food is low, so it will take longer to complete.
Alternatively, I could build a worker first, set it farming, then build my settler. Workers are significantly cheaper to produce than settlers. Perhaps the extra lead time to produce the worker would be offset by the accelerated production of the settler. But producing both the worker and the farm(s) takes time.
In general, given that I ideally want a settler as quickly as possible, is it a better strategy to build the settler first, or the worker? Are there any other factors to consider?