You should consider the pros and against of no dashes, dashes and underscores from two different perspectives: search engine optmization and usability. I think that from a usability point of view, it is better to get a domain with no underscores and no dashes at all. If your domain uses one of them, you will find yourself in the future always specifying whether it is a dash or an underscore. From a search engine positioning perspective, it is true that Matt Cutts used to reccomend dashes until lately when he said that Google no longer makes a difference between dashes and underscores.

Fernando Maciá