When comparing Amethyst vs Magnet, the Slant community recommends Amethyst for most people. In the question“What is the best window manager for Mac?” Amethyst is ranked 5th while Magnet is ranked 11th. The most important reason people chose Amethyst is:
Amethyst is published under the MIT license making it open-source and id offered completely free. This allows anyone to give the tiling window manger a try, which is great.
Specs
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Pros

Pro It's free and open-source
Amethyst is published under the MIT license making it open-source and id offered completely free. This allows anyone to give the tiling window manger a try, which is great.

Pro Recently updated and rewritten in Swift.
The application was written in Objective-C, but was recently updated and completely rewritten in Apple's new native language Swift.

Pro Works with keyboard shortcuts, no mouse needed
As it's designed to work closely to how xmonad, Amethyst uses keyboard shortcuts to control the windows being used, meaning no mouse input is necessary. This can make for a faster way to control windows.
Pro Multi-monitor suppport
Pro Has many common layouts to choose from (fullscreen, floating, row, column, wide, etc)
Pro Can switch between tiling and floating mode
Pro Under active development
Pro Configured via UI
No CONFIG file provides security and will avoid making amethyst crash. Instead configured via simple UI
Pro True tiling means never losing track of windows under a huge pile of others
Pro Keyboard centric
Easy to learn keyboard shortcuts.
Pro Intuitive handling
Just drag the window to one of the sides of the screen and it snaps there. This is especially useful for those who can't be bothered remembering the shortcuts.
Pro Cheap
At $0.99, it is not that expensive.
Pro Mouse controllable
Supports dragging a window along the edges of the screen to activate.
Pro Do not have to learn or remember keyboard shortcuts
Pro It's often on sale
For example, at the writing of this pro it's currently at $1.
Pro Up to six external displays supported
Cons
Con Can be buggy at times
Con There are more user friendly window managers available
There are certainly more user friendly window managers available for MacOS, such as Divvy or Moom. Amethyst, however, is more powerful and customizable than the more user friendly options available and therefore may be a better choice for a user looking for more advanced capabilities and customization.
Con Cannot setup custom sizes and window positions
Unfortunately, you can only move windows to preset positions.
Con Can only be purchased from the App Store
Magnet is only available in App Store.
Con Can't hide the menu bar icon
It would've been nice to be able to hide the icon so it doesn't clutter the menu bar.
Con Interference when switching between folders
Default Folder-X app "warns" that Magnet App may interfere with it's ability to switch between folders in Open and Save dialogue boxes.
Con Causes cursor bugs in IntelliJ IDEs
Unfortunately, Magnet causes the caret to bounce around in IntelliJ IDEs, this is very annoying.
