Both options have a lot of competence, I mean that you can find a lot of websites with both keywords. My advice is to choose another one.
Are you asking about keywords on domain? If thats what you mean. I prefer to use keywords with dash to separate keywords. Using keywords on the URL can help you with your rankings but your rankings would still depend on how you optimized your content with your targeted keywords.
Hello, As say Matt from Google, - is better than _ Look here http://www.mattcutts.com/blog/dashes-vs-underscores/ Best, jakomo