When comparing Growl vs ShiftIt, the Slant community recommends Growl for most people. In the question“What are the best power user tools for macOS?” Growl is ranked 15th while ShiftIt is ranked 46th. The most important reason people chose Growl is:
In addition to all the applications that other people have written that support Growl, Growl includes several 'Extras' that serve various functions.
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Pros
Pro Support for a lot more applications than the default notification system
In addition to all the applications that other people have written that support Growl, Growl includes several 'Extras' that serve various functions.
Pro Ability to be themed
Growl comes with almost 20 themes pre-installed so you can pick what works best for you. You can also create and install your own custom themes.
Pro Sync with mobile phone via Prowl
Prowl is the Growl client for iOS. Push to your iOS device notifications from your Mac or Windows computer, or from a multitude of apps and services.
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 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.
