Hi
I do most layouts using Designer and loads mostly data from code.
But you dont have to choose one way over the other.
The UI forms is converted to code ( see setupUI() )
so sometimes i use Designer to create a layout/setup and then use the code from my code as templates.
The main reason we like using UI files is that it allows the non UX developers to do a bit of GUI sometimes as its less involving to drag a new button to
a layout than reading heaps of setup code to find right spot.
But basically, its more a matter of taste than anything technical . But there are things you need to do from code as designer cannot do it.