IMO keywordkeyword.com is the better domain name. keyword-keyword is the better folder name and keyword-keyword.html is the better page name.
Of course, keywordkeyword.com is better than keyword-keyword.com; but keyword-keyword.com is also good if keywordkeyword.com is already taken. Get it?
It depends on which angle you look at it. For search engines like Google, they prefer to have the hyphenated domains. Google thinks that it makes more sense to the internet users. but if you want to get some free traffic, then the unhyphenated domain will be better, this is because people will not usually type domains with hyphens when they are searching for information on the internet. But for me i really prefer keywordkeyword.com it looks kind of better to the eye and easy to type on the computer keyboard.
That depends on the length of each keywords you are adding in your domain name, if it is longer, I think you should go for brandable and short domain name.
Generally people never use or remember hyphen in domain name that's why domain name without hyphen is more fruitful. but if you don't get domain without hyphen then you can use hyphened domain. There will be no any drastic change will occur. I have both type of domain name and both are doing well on google.
Try to go for the non hypenated one. Unless it is expertsexchange (experts-exchange, but what did you see? expert-sex-change?)
I also came across some reviews which told that hyphens are not good as a separator for internal pages. Is That True? They say that instead of hyphens underscore are preferred by SE. For example: http://www.yourdomain.com/new-page.html is not good in terms of SE. Instead of that you http://www.yourdomain.com/new_page.html is more preferred by SE. Please let me know the fact.