In valve industry the most common material is carbon steel. While brass is an excellent general-purpose metal you can use in a broad range of plumbing jobs, stainless steel has other applications that might make it the superior choice instead. For one, stainless steel valves are much better in terms of corrosion resistance.
In general, stainless steel is going to last longer than brass. It has greater durability and acidic resistance. However, brass has better corrosive resistance and ductility. Because of that, stainless steel is of better quality in construction applications.