I don’t know, but categories are a HUGE boost in the Googlejuice, so that’s what I use. I could explain why, but just take my word for it. Don’t use more than a dozen combined tags and categories on any particular post, though, or you could get tossed off the Global Category Pages, which is not good.
In one of the threads. I heard it second hand first, but Mark did say using more than that can get your blog bounced off the global pages. Not too late to delete some, though. And it’s not a hard and fast rule: you’ll see some that have more. No idea why there’s a difference.