Categories, tags, or both. What's best for SEO?

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I'm wondering if anyone here had any thoughts about whether I should use categories, tags or both for SEO?

I've been doing some searching and I found one site that said tags are better if you're adding several new posts each day, but if you're only blogging a few times a week or a month, then categories are better.

Do you guys agree? Should you ever use both?

If it matters, I'm using Genesis and the Copyblogger theme. I typically blog a couple times a week, and I've only been using categories.

Thanks.