Better experience or better visual design. What matters the most when designing?

Both matter. But a higher priority goes to experience. If you can deliver a great experience to the app/product, you can be guaranteed of a successful product. Designing for users is the main priority, after which visuals come into play. Making your product usable is what matters than the jazzy UI and design. You can survive with average UI if you deliver on experience. But if your experience fails, your whole product/app fails. There needs to be a balance between both, great experience and good looking UI. Experience takes a bigger chunk of the cake; because without that, there is nothing to really show. Design and focus on the experience, then the visuals and everything will then fall into place.