It isn't even up for debate among anyone with in depth rendering knowledge. Cycles is free. That's the extent of its advantage. Renderman is faster, far more flexible, has true layered shaders, handles volumetrics, lots of geometry types, displacement, ptex textures, has better integrators, AOVs, a shading language, a detailed API, light path expressions... There are probably twice as many things I can't think of at the moment.