It seems like both commands do the same thing?
When you release the assigned key...
workbench.action.quickSwitchWindow switches to the selected windowworkbench.action.switchWindow keeps the popup open until press enterSee also the version 1.14 announcement:
[...] a new command
workbench.action.quickSwitchWindowwas added to quickly switch between opened windows. Press and hold the modifier key to cycle through the list of windows and release all keys to open it.