Hyphens are prefered by Google as word separators in URLs, NOT underscores. Matt Cutts has discussed this extensively over the years. Here is a post from August 2005 where Matt explains how Google saw the hyphen then MUCH differently than the underscore http://www.mattcutts.com/blog/dashes-vs-underscores/. Even as late as August of 2007 Cutts states that they were 'looking' at treating them the same, but still treated them differently at that point in time in this post http://www.mattcutts.com/blog/whitehat-seo-tips-for-bloggers/. Note the 5th-7th paragraphs paying particular attention to paragraph 7: So late 2007 Google STILL was not treating underscores as word separators. Even today the Google Webmaster Help documentation lays it out very plainly at http://google.com/support/webmasters/bin/answer.py?answer=76329&topic=15261 that Google still prefers hyphens over underscores. Note the second paragraph: There is likely STILL a reason for this... I have yet to see an official announcement from Google that they now treat the underscore as a 'real' word separator equal to a hyphen throughout their systems. The closest thing to it was Matt's comments at the Wordpress conference in July 2007 which he clarified in the August 2007 post above where he said they were looking into it... but had not yet done it. If you are starting a new site, I would suggest avoiding underscores and using hyphens instead as word separators until you hear an official announcement about it.
Thanks for all the input everyone.The consensus seems to be keywordkeyword if you can get it but using a (-) is almost as good and never use underscores. Nice post fluxos
I can tell you from experience - a two keyword domain name w/dash (i.e. keyword-keyword.com) is becoming higher in demand as all of the generic .com's are being bought out. I have a few dash separated 2 keyword .com's that get type-ins every day. Then again, those type-ins could be coming from domainers who want to see what's there
In the eye's of Google, the second one it will like better, although there's not too much to choose between both of them, but yes slightly better on the second choice.
Honestly speaking, Many webmasters avoid using hyphens in domain names because visitors might get confused and enter the wrong URL.