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I've been having a lot of trouble with MR in Revit. They've taken away some features that are available in other packages. There's little control over light sources - specifically the sun. It handles materials with an image map in the diffuse channel differently that a generic material (or standard material, if you were in MAX years ago).
Blender has open source unbiased physical light rendering engines which plug in. Yafaray, luxrender, indigo, etc.
After not finding what I'd hoped for in Revits MR, I've been looking at ways to get my models into Blender as well.
Buying MAX would be nice, but as a startup company and one man show, it's not an option yet.