When comparing iA Writer vs Letterspace, the Slant community recommends iA Writer for most people. In the question“What is the best Markdown editor for OS X?” iA Writer is ranked 11th while Letterspace is ranked 24th.
Ranked in these QuestionsQuestion Ranking
Pro Can tell how long it would take to read through what you've written
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 Create whole document by including separate images/markdown docs
This is new and very awesome feature in iA Writer 4.0
Pro Markdown support
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 Available for all Apple devices
iA Writer is available for Mac, iPhone and iPad.
Pro Can export a Word document
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 Advanced Markdown support
Features Markdown support that many other text editors don't; such as Todo's ( -[x] ) and Strikethrough.
Pro @mentions and #tags for easy organization
Pro Simple, yet powerful interface
Keyboard shortcuts for basic text formatting. A few theme options. Full screen mode and swipe gestures.
Pro Full featured iOS app
This was a iOS first app and that is clear. Great swipe bar interface on iOS.
Con No Windows or Linux support
Con No in-app Markdown preview
You can export to PDF or HTML with formatting, but no preview in the Mac app.