Robots.txt vs Rel=Nofollow vs Meta Robots Nofollow : SEO Book.com
Aug 6, 2008 - Aaron Wall's chart comparing elements
SEO hard to understand
I was just fixing up our Robots.txt tutorial today, and figured that I should blog this as well. From Eric Enge's interview of Matt Cutts I created the following chart. Please note that Matt did not say they are more likely to ban you for using rel=nofollow, but they have on multiple occasions stated that they treat issues differently if they think it was an accident done by an ignorant person or a malicious attempt to spam their search engine by a known SEO (in language that is more rosy than what I just wrote).