Not sure whether to use dashes or underscores in your website URLs? There is a difference, take a look at this video from Googles Matt Cutts:
First of all, what is a URL? URL stands for Uniform Resource Locator, and it’s just a fancy term for a web address on the Internet. For instance, http:://www..example.com/some-keywords-here..html, is an example of a URL. By typing a valid URL into your browser, you would be sent to a specific page on the Internet.
Dashes or Underscores
To understand why it matters, we have to know how the search engines handle these special characters in URLs.
According to Google, they treat dashes and underscores differently. Dashes are seen as a space between two words, while underscores join two words.
For Bing, they treat dashes and underscores as the same thing – a space between two words.
Why it Matters
The topic of URLs have been discussed repeatedly over the years in the SEO community, and the consensus seems to be that keywords in URLs do matter at least a little bit in terms of search engine rankings. And although Matt Cutts tells us not to lose sleep over this, we can still come to the conclusion that a pretty URL structure with clearly separated keywords (dashes) is better than an ugly URL structure with no keywords. Additionally, a pretty URL structure is also easier to remember and can result in higher click-through rates.
Here is our advice. If you already have an established site using underscores in the URLs, do not worry about changing them over to dashes as the benefits are minimal. However, if your URLs are ugly, long, and using session variables, you may be want to consider doing a URL rewrite. Also, if you are starting a new website, implementing a good URL structure is the way to go.
My personal opinion is go for the Dash as the underscore could make you the underdog.