There is no advantage to freewheel over freehub. The freehub design improves on so many things and is really one of the great innovations in cycling in the past twenty five or so years.
However, if you've got an old bike and old components, stick with a freewheel. If you're going to use your five or six speed components on a freehub wheel, you're going to have to custom build yourself a cassette from assembled cogs, and then deal with spacing problems from using too few cogs on a cassette hub. None of that is a huge deal, but it's not worth dealing with if you have a working freewheel, or can get a working one for dirt cheap on eBay or in the back room of any bike shop.
If you're planning on upgrading to modern STI or other compemporary components, then a freehub will be your only option, and asking is a moot point.
Last edited by bonechiller; 10-22-08 at 09:41 PM.