Dashes (Hyphens) vs. Underscores – Which is better for SEO?

I can’t find a solid answer on this potentially important SEO topic.

The most referenced source seems to be this blog entry from Matt Cutts (a Google employee). That entry, however, is from 2005.

That’s ancient in internet terms. That article advocates for dashes over underscores, but it’s based on rather flimsy data.

There’s a follow-up entry of sorts from August 2007 where Matt seems to say underscores are fine, but dashes are slightly better.

And that’s it. I can’t find any definitive information from either side of the fence since then. That’s over a year and a half ago.

Arguments (from people who have some evidence to back it up, as well as people who are just guessing) on the side of dashes:

  • It looks better.
  • Dashes are used in everyday language (i.e. natural language).
  • Sites with dashes in their URLs show up higher in search results than sites with underscores.

Pros (again, from people who may or may not know better) for underscores:

  • Underscores are not used in everyday language, so they’re a good replacement for spaces.
  • Plenty of sites (Wikipedia, TypePad, etc.) use underscores.
  • Sites with underscores in their URLs show up higher in search results than sites with dashes.

Ugh. Can’t anyone provide a definitive answer? I have sites to build!