Not to split hairs here, but most people are trying to drive business by way of their web sites...not all but most. And most are trying to offer some sort of quality content. So if you are trying to sell something -- say your services... then one site is the way to go.
However, if you are some sort of affiliate marketeer then that's another issue and the more sites the better at times. Generally one site is best to build up and easier to
seo, but it's not completely a black and white issue.
Good luck with what ever you decide!