It comes down to whether you already have something to test.

Usability testing needs an artefact. A live product, a prototype, even a rough sketch of a flow. You put it in front of someone and watch where they struggle. It answers “does this work?”

Customer research doesn’t need anything to exist yet. It answers “what should we build, and for whom?” You’re learning how people currently solve the problem, what they use instead of you, where the frustration sits, and what would need to be true for something new to be worth switching to.

The mistake we see most often is teams reaching for usability testing when the real question is a discovery one. You can test a checkout flow to perfection and still be selling something nobody wanted. Testing tells you the door is hard to open. It doesn’t tell you whether anyone wants to go through it.

A rough rule: if your team disagrees about what the customer’s problem is, do customer research. If you agree on the problem and disagree about the solution, test it.

If you’re not sure, start with a small discovery round. It’s usually cheaper than building the wrong thing well.