Hyphen. Google sees hyphens as dividers in URLs and ignores underscores. cause when you y\use underscore '_'. Google will combine the two words on either side into one word.
Its better to design your url without hyphens. And it would be easy for the end users for remembering this.
My vote goes to without hyphen as it is easy to remember or to tell somebody...I don't think so it will matter. At the end your work is matter.......