subdomain vs domain vs directory

Hey *!

I am planning to make some article site like ezineerticles, so my site will be about anything...
What do you guys think would be better for seo?

article-topic.example.com/article123.htm
example.com/article-topic/article123.htm

(of course article-topic.com would be best but i am tired of buying domains or should I?)

thanks.