Performance and Coding
As such, Qt Widgets could be considered low-level compared to Qt Quick. But this implies that in the long run, a Qt Widgets project will run faster and have better performance. Being low-level can be good though, as Qt Widgets is more exposed to native API (the QtCore module, Qt Style Sheets, etc). That said, it is often used for desktop development.

Qt Quick caters more to mobile development (although it can still be used in desktop development). It has ready-to-use popups, animations, tabs and layouts, flickables, drawers, and the usual controls; all ubiquitous in mobile development.