Ecto does offer full rich-text editing, which Mars Edit doesn't seem too. That's kind of a bother, but possibly not enough to matter.
UPDATE: Hmm. That was...unsatisfactory.
UPDATE: Uploaded an image to Flickr from iPhoto. Inserting via Mars Edit's media manager.
UPDATE: Better, though I just discovered I can accomplish the same thing with ecto. I kind of think I like Mars Edit's interface better, tho.