This question has a lot of good answers at the startup stackexchange site. http://answers.onstartups.com/questions/3822/domain-names-with-a-hyphen-good-or-bad-idea Here are the highlights
[If you go with one word,] you better make sure your "one word" name does not have possible bad connotations without the hyphens. e.g. www.experts-exchange.com would be VERY bad as one word: www.expertsexchange.com because some people would see: "Expert Sex Change".
Search engines really really really like to see keywords in domain names. It's probably more valuable to rank higher in search results than to eliminate a hyphen.
I used to own the domain name Webmaster-Resources.com . WINDOWS Magazine, which had 1 million subscribers at the time did a full-page write-up. They left out the hyphen. Ouch!
[A hyphenated name] conveys inferiority (especially to the site w/o the hyphen in their domain) in that you accepted a hand me down domain. In addition it confuses users as to if your site is affiliated with the hyphen-less site. It looks junky and spammy. When I see sites like las-vegas-info-site.com or dot-net-developer-tips.com I think spam site. As a matter of fact the more hyphens in the domain the more it subconsciously conveys "spam" and "avoid site" to the human brain (mine anyways, by an exponential factor).
Overall, the advice was to spend more time finding a memorable, spell-able, brand-able name, than worrying about whether or not you put a hyphen in it.