References from a real comparison. Computer World's Matt Lake puts Tumblr and Posterous through a simple website build to test the pros and cons of each microblog platform.
"To evaluate Tumblr and Posterous Spaces, I used a conceptual design for a blog site, along with graphics and editorial content, and tried to make the sites do what I needed them to. The proposed site is called HappyFlight; it has a logo, two contributors and two static pages ("About" and "Tips and Tricks")." -- Matt Lake