When comparing Slate vs ShiftIt, the Slant community recommends Slate for most people. In the question“What are the best power user tools for macOS?” Slate is ranked 21st while ShiftIt is ranked 46th. The most important reason people chose Slate is:
Slate is similar to Linux WMs like Xmonad in that it is nearly infinitely configurable. You can either use the default configuration file or just Javascript for very complex/dynamic settings. Slate also offers editing a ".js" or a ".slate" file for configuration.
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Pros
Pro Extremely configurable
Slate is similar to Linux WMs like Xmonad in that it is nearly infinitely configurable. You can either use the default configuration file or just Javascript for very complex/dynamic settings. Slate also offers editing a ".js" or a ".slate" file for configuration.
Pro Based on hotkeys
Hotkeys can be set to re-size and focus windows, as well as activate layout presets.
Pro It's free
Slate is completely free to download and use, which is a great option in a field of Mac WMs that often costs money to use.
Pro Allows for tiling
Slate allows for customizing the config file in order to emulate tiling windows.
Pro Lets you set default layouts
By using a feature called "snapshots", users can set up different default window layouts in Slate that they can switch to and from on the fly.
Pro Window hints
Slate offers window hints which are an intuitive way to change window focus.
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 Free
Pro Intuitive
ShiftIt is very straight-forward to use for most frequent usages.
Pro Open source
Pro Configuration can be committed to your personal dotfiles repository
Configuration winds up living in .hammerspoon/init.lua and .hammerspoon/Spoons/ShiftIt.lua. Very easy to track configuration changes.
Cons
Con Project is no longer in development
Development is no longer active on this project as of 2017.
Con Hard to configure
There's no graphical user interface for configuring Slate, meaning it has to be done from the command line interface or by editing a config file. This makes it somewhat difficult and time-consuming.
Con Missing features
Unable to perform all tasks of software it was meant to replace.
Can not move windows between 'Spaces' (virtual desktops).
Con Video fail
Demonstration video failed to highlight any of the advantages of the windowing system. Or at least, the advantages were so subtly intrinsic that no-one who didn't already use the system could appreciate them.

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.
