Of your 2 examples, the right anwer is "CUAL". Generally, you use 'QUE" when you are refering to things in general, and when you wish to ask for a specific group of things you use "CUAL". For example: 1. ¿Qué película has visto últimamente?"= What movie have you seen recently? 2. ¿Cuál es tu tipo de película favorita?= What is your favorite type of movie?

1) ¿Cuál es el capital de Paris? (What is the capital of Paris?) 2) ¿Cuál es la dimensión de esta casa? (What is the dimension of this house?)

"cual" para ambas preguntas

Look at these examples, hope they clarify: Que quieres? What do you want? Cual quieres? Which one do you one? "Are there instances where these 2 words are used interchangeably?" No, not really

On the other hand, 'What' doesn't always means 'cual'... for example, your question should have been "CUAL es la diferencia entre “qué” y “cuál”?"