James Williams blogged a quick comparison of Artifactory to Nexus. He leans toward Nexus, and his post provides a good feature comparison between the two repository managers. Competition is great, and Sonatype really does see the presence of other strong repository managers as a sign of a healthy community. We believe that Nexus is the best option out there, and it is nice to see people notice all the hard work this team has put into making a solid repository manager.
Read his evaluation and comparison here: http://cfossguy.blogspot.com/2008/09/my-artifactory-versus-nexus-experience.html