Categories vs. Tags: Who wins?

When I different productivity levels for categories and tags, which is counted? So if a site is categorized as, say, "News/Blogs," which I have set as -2, but I tag the specific site as "School," which I have set as +2, which wins? Do they zero out? or should I set all Categories to 0 and rely on tagging?

Thanks! I love the tracking, just want to get a better sense of my productivity score!