Is QT Quick better than QT Gui for Desktop applications
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Use Qt instead of QT (short for QuickTime)
As for your question - it depends on what your application is. In general, QML is faster to prototype and develop, but due to the limited number of components it may actually get slower if you have to implement ordinary GUI elements in QML, whereas with QtGui you have a ton of component widgets with tons of ready to use functionality.
I would go for QML if I wanted a more custom look, which BTW is achievable with QtGui as well, even thou it will be more cumbersome. If you want a traditional GUI application with push buttons, check boxes and so on, then QtGui is the definite way to go.
With QML in Qt Quick 2 you are likely to get higher rendering performance, as for Qt Quick 1 - it uses the same rendering pipeline as QtGui, and will actually perform a little slower because of the slower JS binding of properties. With QML you might have runtime interpretation which is slower than QtGui which compiles to native instruction code.
If you have a small app, sort of "mobile device application" QML is the way to go, but for big and serious desktop programs QtGui is more adequate, and you still have the power to achieve highly custom looks with subclassing, styling and paint event overriding.
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For QML, you do have some basic components : "Qt Components on Gitorious":http://qt.gitorious.org/qt-components/desktop
Here was the post about it : "Qt Labs blog":http://labs.qt.nokia.com/2011/03/10/qml-components-for-desktop/