Re: Categories vs Structure
Personnally, I would suggest using both. They are not really the same thing. A structure can be thought of as the chapters of a book. They are only ever in one place at a time. Categories on the other hand are much more flexible, and can be thought of more as a kind of "index of ideas". The same page can be in multiple categories, and multiple categories can point to one page.
So it depends a bit on what your site is like - if there is a lot of information that is confusing to navigate, then categories will be a great help.
Be warned though: you need to think carefully about how you set them up!
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