I've been using QML for the past six months, and I wouldn't touch QWidgets with a 10ft barge pole now. The relative ease/difficulty depends on what you want to achieve. QWidgets are simpler if you want to take an off-the-shelf approach, but customisations are easier using QML, especially if you want to add transitions. You can also create re-usable QML components quite easily. A re-usable push button only requires a few lines of code. Connecting a QML UI to existing C++ code is also quite straightforward, so you can use your C++ data models etc.