From Middleman's site: "Middleman ships with support for the ERb, Haml, Sass, Scss and CoffeeScript engines." I swap in a middleman-slim plugin, though.
So it seems that Middleman offers very little than the latest version of Jekyll, though I did read someone say Middleman is more focused on pages as opposed to Jekyll being very focused on a blog/posts.
I haven't used the latest version of Jekyll, but from having used older ones, I can say Middleman definitely feels more flexible to me, in terms of how its configured. See my comment elsewhere in this subthread for more color on that.