I want to create a Desktop app that runs on Windows (No problem if it's able to run on other devices but my main focus is windows). It needs to have a great design and looks pretty nice.
Can you please tell me which tool is better when I'm the only person designing the whole app?
Your question boils down to a decision between two programming languages, and it's hard to give advice for you. It heavily depends if you already build up a huge skillset in one of the mentioned languages.
The Qt framework is known for its nice and structured code, documentation, and available examples. Also, if you think about performance Qt and modern C++ (11,14,17) is your friend. As you referenced "great design and looks pretty nice" you should definitely have a look at Qt's own solution to address fluid and modern user interfaces for mobile and desktop devices: QML.
Javafx is actually quite good, and feels modern. Some widgets are quite hard to master though (for example the table view...).
I have written a low-code development framework ( Open Lowcode ) based on Javafx. You may want to have a look if it can help you. It is purely open source.
