Neither method is automatically better. The stronger approach is the one that fits the information available and lets you explain the market logically.

A top-down approach usually starts with a broad population or spending pool and narrows it. A bottom-up approach builds from units, locations, customers, capacity, or transactions. The market-sizing guide covers both approaches in more detail.

Choose one, state why it fits, and keep an alternative in mind as a cross-check. Interviewers care more about transparent assumptions and coherent logic than about using a particular label.

I usually pick whichever side has assumptions I can defend better.

Cross-checking with the other method is great if you have time.