For what purposes? I'd always use the second one in all instances. It's filename independant, and represents a logical directory structure more.
1st. www.keyword.com 2nd. www.site.com/keyword/ 3rd. www.site.com/folder/keyword.htm and then a lot of things like www.site.com/kewyord/keyword.htm www.site.com/main-keyword/second-ketword.htm and things like those