Google News, Inside.

Dashes, dashes, dashes.

This is what the official statement from Google says. (link)

Consider using punctuation in your URLs. The URL http://www.example.com/green-dress.html is much more useful to us than http://www.example.com/greendress.html. We recommend that you use hyphens (-) instead of underscores (_) in your URLs.

So when you’re thinking about using dashes or underscores in your permalinks, use dashes.

Let’s take a quick look at the blogging/CMS platforms and how they handle this.

Dashes or Underscores?

Blogger: dashes
WordPress: dashes
Movable Type: underscores
Expression Engine: underscores
Drupal: N/A (uses /node/###)
Joomla: underscores

Based on that quick analysis, Blogger and WordPress are more SEO friendly out of the box. I personally recommend WordPress, both self-hosted and WordPress.com hosted, for a number of reasons, but mostly because it’s so flexible. But if you’re addicted to Google, you might want to try Blogger.