Gambrel. The curved profile creates significantly more overhead volume, which is why most lofted barns use that style. You get a taller, deeper loft and more headroom on the main floor compared to a gable (A-frame) roof at the same building size.
A gable roof has a cleaner, more traditional look and works great on utility sheds and garages. You can still add a loft to an A-frame building, but it won't be as spacious as what a gambrel gives you. Both are solid, well-built options. It comes down to whether you want maximum interior volume or a more classic profile.
