yeah, i wasn't talking about IPR. Perhaps knowing a bit more about mental ray is your cheaper/better alternative.

Besides, are you rendering passes and compositing? or do you just rely on a primary render from maya? This is all dependent on your workflow/pipeline.
I render in mental ray/renderman and output passes which are comped in Nuke. This is the process and it'll take a similar amount of time regardless of the renderer. So for me the difference is, do i want 3D motion blur or vector blur? can i raytrace shadows? can i render with displacement or do i have to use bump? how much memory are my frames using. etc etc
so in a way it makes no difference.