What directory scheme should I take?

Hello folks,

This one is for anybody who has been working with directories and is more aimed to a marketing strategy. What do you think is better, for example I want to create a directory of mechanics. What should I go for: creating one directory (domain) for all kind of mechanics, or one domain for cars, another for bicycles, another for electronics, and so on. Anyone who has some experience regarding to this, please respond.

Thanks,
Marco