I have to agree with MikeSchinkel on this being an opinion question so here is my opinion.
I would go in a different direction:
Yes Use under 10 directories and use tags all over but do use sub-categories.
The beauty of WordPress build-in categories is their way of organizing posts in hierarchy,
and judging by geniustypes.com they use that same structure of about 3 main categories and there rest are subcategories.
To be honest I can't say witch way is more effective user wise but in your case the management of the categories will be a lot easier and no plugin is needed because WordPress
category management that come's out-of-the-box is more then sufficient for your needs.
Now again this is my opinion that i have picked up in years of experience but I'm more then sure that MikeSchinkel has (probably) more experience then me and I respect and understand the way he is suggesting.
hope I helped.