Ok vitaplease, but thats just for Google. After the last update and alot of peope saying that they were hurt by Google in some way or another, I would like to know if this is the same with all search engines!
Most engines treat hyphens and underscores the same. Google does not. So given the fact that Google controls almost all search on the entire web, I would tend to go with the one that works the best on Google. (Even if I wasn't doing well on Google at the moment).