There are various arguments on whether to use underscores "_" or hypens "-" as dividers in your URLs - most of them aesthetic. However, to my way of thinking, one fact decides the whole argument. Google sees hyphens as dividers, it ignores underscores. This means it's more likely to extract keywords out of your domain, directories and filenames if you use hyphens to separate your keywords, and hence your webpage will rank higher when searching for those keywords.