I'm wondering this as well. I've really enjoyed Jekyll in the past and the latest version of Jekyll has support for Sass and CoffeeScript compilation out of the box now (not that it was hard to add before, but nice to just have now). I would also like to know about what Middleman offers.
I must admit, one thing about Middleman that makes me roll my eyes is the use of "Hand-crafted" in the description of the project.
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.
I must admit, one thing about Middleman that makes me roll my eyes is the use of "Hand-crafted" in the description of the project.