The hyphen is more common and more popular. But does it really affect anything? If its intend for seo/google, i think its the same. But (-) is better.
in seo google read underscore _ for separator better it was google algorithm of four years ago i should do some research about now
Both are good but i think "-" (dash) is better coz when we use underscores("_")then it's looking like space .anyway "-" is better for site address.
There are many reasons why a hyphen is better than an underscore. Here is a small screen capture from a page I am preparing on URL Design and File Naming Guidelines: http://www.typosandmisspellings.com/gifs/hyphen-vs-underscore.png