Tags vs. Categorization

My usage of tags vs. categories for content is cyclical.   Is it easier to provide a tree or breadcrumb structure to find new topics or allow for many to many associations?  At the moment, I am using tags again in my email (thanks to Jamie!) vs. moving to folders...the latter seemed too old school.Tagging also allows me to change what I think of the subject by adding a tag or deleting but not messing up the structure.  Kind of like MongoDB vs. SQL.