I'd stay with Mental Ray if I were you..tested both renderers.....Renderman is really awesome, but Mental ray is much cheaper for us MAYA users at the moment...also Renderman only supports multiple processors in one system since version 11.5.....Mental Ray supports multiple processors since the beginning I guess.

In Testrenders Mental Ray is faster in every aspect...Raytracing especially and even scanline Rendering....Renderman has the fastest motion blur available on the market at the moment..other rendering systems are not even touching them...only a plus of 20 % of Rendertime due to Subpixel displacement...

Renderman is the renderer that uses less Memory during rendertime...therefore it's perfect for heavy scenes, other renderers are not even opening.

Well...if you want to know some In deep Detail...just ask...can't write everything here right now...

cheers

b u l l e t