arXiv:2606.13843v3 Announce Type: replace Abstract: Generative user interface (GenUI) tools use large models to create UI mockups from high-level descriptions; a recurring interaction paradigm starts with an unstructured prompt, followed by depth-first generation of a polished, high-fidelity prototype. To examine how it supports early-stage, 0-to-1 design exploration, we contrast this paradigm with structured input, breadth-first workflow, and low-fidelity generation. In our study, 24 industry professionals in early-stage product and UX design used a commercial GenUI system and our research prototype as study-era instances of each paradigm for a comparative evaluation. Findings show that 1) structured input surfaces key facets but raises entry barriers; 2) breadth-first workflow reveals more possibilities, but previewing multi-screen UX ideas remains hard; and 3) despite low fidelity's value, participants rated high fidelity as better suited to their design stage and to communicating ideas. Our findings contribute a model-agnostic, design-level account of how GenUI users specify intent, traverse alternatives, and interpret fidelity.