What is best to use in a web page bookmarking tool: tags, a hierarchy or both?

I am creating a web page bookmarking tool. Traditional bookmarking has always given mechanisms to organize our bookmarks into a file/folder structure. New paradigm of flat tagging systems have now come too. I really liked the ease with which we can add tags to something but I feel more organised when using the file/folder system. So I have been thinking hard on being able to achieve best of these both: a hierarchy of tags. But can some one put an argument in the context of bookmarking and UX as to which approaches are better and why? An example in working (like existing application) would be a great help too.