When comparing iA Writer vs Zettlr, the Slant community recommends Zettlr for most people. In the question“What is the best cross-platform note-taking app?” Zettlr is ranked 49th while iA Writer is ranked 50th. 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
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Pros
Pro Noise Free Writing Experience removes even the program bar
The Noise Free Writing Experience is one of the first approaches to create a first class distraction-free environment. The feature hides the Mac program bar when you start writing.
Pro A mode that brings focus to the sentence you're working on
Enabling Focus Mode will dim the whole text except the sentence you are currently writing on.
Pro Can tell how long it would take to read through what you've written
Pro Markdown support
Pro Syncs using iCloud and Dropbox
iA Writer can sync documents using iCloud or Dropbox from within the editor for use across multiple devices.
Pro Create whole document by including separate images/markdown docs
This is new and very awesome feature in iA Writer 4.0.
Pro Can export a Word document
Pro Available for all Apple devices
iA Writer is available for Mac, iPhone and iPad.
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 No Linux support
Con Obtrusive, like someone WITH CAPS LOCK ON
Too loud, too much going on, and definitely an in-your-face sort of feeling.