Thanks to it's popularity, Jekyll has a large and active community of users. This means there is plenty of learning material available for Jekyll and it's easy to find help from other users when needed.
Middleman is a big piece of software, it's not simply a static blog generator. Because of all the functionality and flexibility it offers it can be a little more complex than other static site generators and a little harder to learn all of its bells and whistles.
As Octopress is simply a framework on top of Jekyll, it benefits from being based on one of the most popular site generators available. The primary benefit is that it is backed by git and is built in such a way that the site can be easily re-generated if the content changes.