I may be cheating a little by not answering the question precisely. The Ulysses screenshot does look beautiful. It does offer a clean user interface. The typography does look great. It combines the Markdown “source code” with layout. Ulysses goes quite far and almost hides the Markdown markup. It still is visible, but in a subtle way. If you like this way of writing, basically putting the layout to the foreground while not seeing too much of the source code, it is a very good option.
iAWriter is putting the Markdown markup in the middle of the stage. You already can see what effects the markup will have when you eventually will export the Markdown file to something else. But the emphasis is clearly on the markup, which does not look as beautiful, but it for sure is functional for those who do like this mixture of source code and WYSIWYG.
Both apps have a different approach and do offer a feature set that has different purposes and options.
So, what is it that I am preferring? None of them.
When I am writing Markdown, I am doing so because I want to store it as plain text in the long run, sitting in a folder, accessible to any app. Another use case is that I want to use the text on the internet and additionally as a PDF. If I write in Markdown, I want to see everything I type. Every blank space, every line break and what not. I am not looking for a focus mode or stuff like that. If I had to decide between Ulysses and iAWriter, iAWriter would be my choice because it does emphasize the Markup more. But it still does not show me everything I have typed in a clear and unambiguous way. How many line breaks between two paragraphs? Where exactly are spaces? How many of them?
I prefer a text editor for that. BBEdit does so much and also with Markdown everything I want:
Marked 2 is my tool of choice if I want to see more of the layout stuff (sometimes as a second window beside the BBEdit window). If I already know that my text will end up as a Word document and nowhere else, I do my writing in Word (using the format templates I need for the specific document). This does happen occasionally, but not very often.