Short answer: for splitting the words better to use dashes.
Why?
Just in case, here is a video, where Matt Cutts (former head of Google’s Webspam team) clarifies the difference between underscores and dashes.
Briefly: by dashes you can split words by logic meaning, by underscores you will split them just visually. For example: this-date-is-10_07_1998.
Leave a Reply