QtQuick.Controls 1.4 and QtQuick.Controls 2.1
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QML and Qt Quick
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No, they can be used together.
However, Controls 2 are in general much better (they occupy less memory, perform better), so I would recommend using 2.x as much as possible.
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@Arkning On the other hand Controls 2 have a modern mobile look&feel. If you want traditional desktop look&feel the Controls 1 are better. On a modern laptop or desktop computer you don't have to worry about memory consumption or speed of Controls 1. I suggest playing with the Qt example applications in Qt Creator to see them in action.
But as was said they can also be used together in the same application.