When comparing SizeUp vs Magnet, the Slant community recommends Magnet for most people. In the question“What are the best power user tools for macOS?” Magnet is ranked 51st while SizeUp is ranked 57th. The most important reason people chose Magnet is:
Easy to learn keyboard shortcuts.
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Pros
Pro Keyboard centric
SizeUp has user-configurable keyboard shortcuts for a set of pre-defined actions.
Pro Supports multiple monitors / spaces
Windows can be sent to different monitors and spaces. It can also be aligned as needed.
Pro Simple
SizeUp is easy and simple to use without having to configure.
Pro AppleScript support allows extending functionality
AppleScript permits adding features such as tiling all open windows. Additionally, FastScripts it allows the user to add keyboard shortcuts to AppleScripts.
Pro Menubar dropdown for shortcuts
Learning a bunch of keyboard shortcuts right off the bat may be difficult to remember at first, which is why a menubar shortcut for the app exists with all the shortcuts listed.
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 Nag window
If a license is not purchased there is a nag window (which suspends function) that pops up when loading the app (at user login). The nag pop-up is also summoned at certain time intervals and by performing a certain number of commands (manually or by keyboard shortcut).
Con No on-the-fly split screen window settings
Unlike some other tiling window managers, SizeUp has no way to change the size of split screen actions on-the-fly. Any time a user wants to change how much room on the screen a window takes up when using the split screen function, they will need to manually change the settings, which can be time consuming.
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.