When comparing Quicksilver vs ShiftIt, the Slant community recommends Quicksilver for most people. In the question“What are the best power user tools for macOS?” Quicksilver is ranked 16th while ShiftIt is ranked 46th.
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Pros
Pro Easy-to-understand noun-verb paradigm for creating tasks
Pro A lot more powerful than Spotlight
Pro Makes it really fast to do a lot of actions
There are customizable triggers and capability to execute multilevel commands - e.g. launch terminal app, give the location of script and supply parameters for the script with one command.
Pro Vast extensibility via plugins
A lot of different plugins for browsers, file systems, mail clients, etc.
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.