If you want to develop native-looking or information-dense UIs, Qt Widgets is the way to go. If you want to create a more Electron-like UI or develop something for mobile devices, I'd go with Qt Quick.
It's also possible to combine both by embedding a QQuickWidget[0] inside a Qt Widgets application.
If you want to get an idea about what both solutions provide, take a look at the examples for Qt Quick[1] and Qt Widgets[2]. Especially Qt Quick Controls[3] are useful to see what UI controls are available in Qt Quick.
I'm working on a desktop app in Qt Quick using QML and C++. Happy to elaborate if there's interest.