When comparing Markdown Edit vs Zettlr, the Slant community recommends Zettlr for most people. In the question“What are the best open source Markdown editors for UNIX-like systems?” Zettlr is ranked 9th while Markdown Edit is ranked 25th. The most important reason people chose Zettlr is:
While many Markdown editors don't offer specific support for a certain type of workflow, or offer features for scientific workflows only, Zettlr offers features that help the writing process of journalists or researchers in the arts and humanities. It's a lot more text-focused than most editors.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Free and open source
Licensed under MIT.
Pro Real-time HTML preview with synchronized scrolling
Markdown Edit is split in two panes. One pane is used for writing Markdown, the other shows the rendered HTML as it's written.
Pro Focuses on writers
While many Markdown editors don't offer specific support for a certain type of workflow, or offer features for scientific workflows only, Zettlr offers features that help the writing process of journalists or researchers in the arts and humanities. It's a lot more text-focused than most editors.
Pro Citation support
While it supports a diverse range of syntax (chart, easy image insert, etc.) found in other editors, the great citation support made it possible to write real articles. Citation from Zotero and Mendeley can be inserted easily which is a huge plus.
Pro Almost perfect
This is the best option, still not perfect, there are some bugs like creating / editing tables and resizing images, but the PROS destroy the CONS, easy quotes, WYSIWYM , attachments tab (supports attaching and opening links to any file), table of contents, TAGs, easy hyperlink between files (same as citations), export to many formats (like Word, HTML5, PDF)...
Pro Renders math in-place through KaTex
Cons
Con Dead project
Con Covers taskbar when maximised
When maximised, the window covers the taskbar completely even when the taskbar is set to be on top of other windows.
Con Does not support pasting images from the clipboard (unless online)
This editor boasts that it's a desktop (i.e. offline) markdown editor where you can copy and paste images directly in the editor which is a feature that many other markdown editors struggle to implement. Unfortunately this is misleading, since the only way to paste an image in the editor is to be online. If, like many of us out there, you want to work offline, this feature will be broken as it requires an internet connection, which is weird for an editor that boasts being a desktop app!!
Con Obtrusive, like someone WITH CAPS LOCK ON
Too loud, too much going on, and definitely an in-your-face sort of feeling.
Con Large, slow, can't open just a single .md file
