This is not easy to answer. QML is nicer for some type of application and worse for others. It all depends on your use case.
Usually the part which is critical for GPU acceleration is a small part of the application and the rest of it should be done with the tool which is better for your goal, so you get it easily done and with smaller total cost.
If you are going to use Qt5 it is recommended to work with Qt Quick 2 applications since these are hardware accelerated and work best.
To find the themos if you already have installed Qt5 the demos and examples are found in :
QT5PATH normally would be something like /opt/qt5/, but it depends where you selected the sysroot when configuring your installation of Qt5.