Ok, so you are building a brand new site and you are trying to decide what your urls should look like? You come to the question what do I do with the spaces? Dashes or underscores?
Should the url look like: /my-great-url OR /my_great_url ?
According to Matt Cutts, head of Google’s Webspam team,
Google treats URLs with underscores as joins.
For example: “/my_great_url” will be treated as “mygreaturl”.
A URL with dashes will separated.
For example: “/my-great-url” will be treated as “my great url”.
Watch this video where Matt Cutts explains.