I have always prefered http://www.domain.com/category/postname
I might as well contribute, too!
I have never used /category/postname, I've stood by /postname from the beginning.
So far it seems to be working well for me.
The first one if you can place keywords that is there in the post in category also
The second one if you cannot do that
Some search engine marketing professionals prefer the domain.com/postname format for seo purposes. They say it's better cause search engines can easily find the keyword in that format. That could be true but I like the form domain.com/category/postname to keep data much more organized.