In case that both are available. mynewdomain.com or my-new-domain.com Maybe it's not a good ex because it's easy to read. but what if. mynameisgeorgebush.com Which will be better to read as : my-name-is-george-bush.com 10x
Most users on the Internet don't know you are allowed to use dashes in domain names. Stay away from dashes or buy both.
Doesn't MSN/live.com place relevance on hyphen domain names more than yahoo and google? Hyphen domain names are better for SERPs right? Doesn't SERPs read the hypens as a space? I realize hyphens are not very good at building a brand...
I would reg both. However, if I had to choose I would suggest you reg the one without the dashes. Good Luck.