When comparing ShiftIt vs Magnet, the Slant community recommends ShiftIt for most people. In the question“What is the best window manager for Mac?” ShiftIt is ranked 4th while Magnet is ranked 7th.
Question Ranking
Pros
Pro Free
Pro Easy to configure
ShiftIt is configured through a simple UI where you simply press the hotkeys you wish to map to a certain action.
Pro Intuitive
ShiftIt is very straight-forward to use for most frequent usages.
Pro Open source
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 Keyboard centric
Easy to learn keyboard shortcuts.
Pro Cheap
At $0.99, it is not that expensive.
Pro Up to six external displays supported
Pro It's often on sale
For example, at the writing of this pro it's currently at $1.
Pro Mouse controllable
Supports dragging a window along the edges of the screen to activate.
Cons
Con A little slow
Similar to Spectacle, but it has a few more features, such as being able to pick the size adjustment increments. Spectacle was a little faster, though. Repeated size changes or moves in succession can cause it to wait for it to complete.
Con Imprecise when readjusting sizes
Con Doesn't support moving windows between workspaces
There's no way to move a window to a different workspace with hotkeys in ShiftIt.
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 Can only be purchased from the App Store
Magnet is only available in App Store.
Con Cannot setup custom sizes and window positions
Unfortunately, you can only move windows to preset positions.
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.