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.
Minification and compression are as easy as setting a few configuration options, and unique asset hashes are available to allow you to invalidate the cache of files that change regularly.
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.
Another advantage of being based on Jekyll is default access to the extensive plugin system for Jekyll. There are also plugins developed specifically for Octopress.
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.
Octopress is designed to remove all the time-consuming and frustrating tasks you usually have to deal with when using Jekyll. You don't have to write your own HTML templates or do any configuration to get started. The default template also takes care of any basic CSS/JS you need to write to get going.