Why should you use dashes over underscores when creating URLs?
SEO experts recommend using dashes over underscores for search engine recognition. Search engines see hyphens or dashes as word separators and sees underscores differently, as word joiners. In addition, dashes have a slight ranking advantage.
From a usability standpoint, URL’s with hyphens are more readable than those without.
The second URL is far easier on the eyes and gives a clear description of what the page will pull up. Since URL’s are underlined, underscores are harder to see and leave returning visitors guessing at your URL. This creates lost users, while hyphens are clearly visible and alleviate that concern.
If you want to rename all of your pages from underscores to hyphens — welcome the salvation of the 301 redirect. Be aware, however, that 301 redirects do create a small loss in link value through the transfer. If you have pages auto-generated by WordPress or another CMS, and you’re getting junk code added, talk to your developer/webmaster about adding some script to the site that will automatically rename those pages to something more user and SEO-friendly.
So what have we learned?
Dashes posses a slight advantage in ranking, are considered word separators and are easier to see, leading to less lost returning visitors.
Underscores are word joiners, are not worth the lost link value to change if it is a long standing site and lack the ranking advantage of hyphens, but still offer a slight advantage over no separation at all.