I know there is many debates on this subject, but still … I can’t make up my mind, so:
If you build a site similar to ebay (just imagine this is ebay), where objects and services can be bought or rented, would you use categories and sub-categories to classify all the objects/services, or rather tags ?
I guess there is no strict answer to this question. A tag or a category doesn’t change the meaning of the object, it is just a feature of your site.
So if you tag a computer as electronics, you can also categorize it as electronics. Tags would add more meta description in addition to categories. Have you used WordPress before? Look at it. Category is central, and tagging is additional description, like keywords. They help in search, grouping, sets, etc.