Neither.

You need to hire a business analyst first who will define what goes into the app and breaks it down to a point where it's granular in terms of workflow, features, functionalities, integrations, algorithms, etc.

This is what sets a foundation on which the wireframes and design is drawn up on.

Secondly, UX is not a standalone function. It's what a UX designer and the lead developer together define in an app, in an ideal situation. I would have the two working together, but only after you nail down the Functional Specifications with a business analyst.