I've read several articles around the web about clean URL structure and how they aid site usability, but I still can't decide on what separator I should be using. Currently I am using _ to separate words in my URL's, but most seem to be using - as the separator. I would appreciate feedback on the pros and cons of the following URL structures;

http://example.com/article/some_article_title

http://example.com/article/some-article-title

Cheers