I use Byword as well. While Desk and Byword are similar in someone ways, Desk has the advantage in terms of blogging support. Byword supports WordPress. Desk will post to Blogger, Tumblr, SquareSpace, TypePad, Facebook, and WordPress. I'm writing a piece on how Desk might be of benefit to people who want to create long form content for Facebook.

Having said that. I think the Markdown support in Byword is much better.