When comparing Spectacle vs Byword, the Slant community recommends Byword for most people. In the question“What are the best power user tools for macOS?” Byword is ranked 33rd while Spectacle is ranked 35th.
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Pros
Pro Free and open source
Spectacle is licensed under MIT with source code available on GitHub.
Pro Simple hotkeys
Spectacle offers easy-to-remember preset hotkeys.
Pro Mouseless
All windows in Spectacle can be controlled with keyboard shortcuts, so no mouse is needed. This makes for a fast way to control windows.
Pro Great UX
Spectacle is easy to use and configure. It's plug and play.
Pro Low footprint
Spectacle only uses up 18-19 MB of RAM, which means there is not much overhead by using this app.
Pro Multi-monitor support
Pro 7+ years of use
Using it for 7 years
Pro Word count support
Pro A dark theme is available
Pro Can publish directly to external services
Pro Can export to html, pdf, rtf
In addition to plaintext, Byword can export to HTML, PDF, RTF.
Pro Exemplary Markdown support
Byword de-emphasizes the syntax itself, while emphasizing its effects. It appropriately adds style, like italic and bold, to text that's designated by markdown and dims the syntax so it does not getting in the way of comprehension. Additionally, there are commonly used hotkeys (⌘b, ⌘i, etc) that can be used to apply style without having to know the syntax or having to type it out each time.
Pro Includes features that speed up writing
For example, while authoring a bulleted list hitting return automatically prepares a new bulleted line.
Pro Available for all Apple devices
Byword is available on a Mac, an iPhone and an iPad.
Pro Syncs using iCloud and Dropbox
Documents can be synced using iCloud or Dropbox from within the editor for use across all devices.
Pro Live update support
Cons
Con Not maintained
The software is free, but no longer seems to be maintained.
Con Impossible to add new functions
While keyboard shortcuts can be changed for the available actions, there is no way to add custom actions which in turn limits the functionality of the app.
Con Limited window sizing capabilities
You can use 1/2 of the screen vertically and horizontally (divide into 4 parts). You can also use the screen in thirds too, but only the left and right third: you can't use the 1/3 in the center. There is no way to do a more granular distribution.
Con Breaks on macOS High Sierra / Chrome or Firefox
Doesn't support side by side windows.
Con Doesn't work with any Adobe products
There are some apps like Adobe Reader that do not work with Spectacle. Basically, any app with a heavily customized UI that breaks OS X accessibility will not work correctly under Spectacle.