I wonder why this discussion always falls to Unity and Godot; Unity source is not available, while Unreal Engine source is.

Unreal also has more features available than both engines out of the box, as far as I am aware. I don't want to sound like an advert, but the shading and Blueprint systems in UE4 are extremely intuitive and easy to work with, and the C++ is definitely managable. I've never used Godot, but the last time I compared Unity to UE4 it felt extremely clunky. Lastly, prototyping stuff in UE4 is fun, once you get used to it and develop your own workflow.