NVIDIA Mental Ray or Autodesk Raytracer? That is the question. I believe that both render engines have their advantages and disadvantages.
The first render engine comparison is of an exterior day render. The NVIDIA Mental Ray image is on the left, while the Autodesk Raytracer image is on the right. Even though these two renders were created using the same quality settings, sun settings etc., one can clearly see that the Autodesk Raytracer engine saturates colours far more than the NVIDIA Mental Ray engine.
When comparing an internal night render, one can see that the NVIDIA Mental Ray engine is far more "realistic" than the Autodesk Raytracer engine. Pay attention to the floor lamp on the left. No electrical, nor photometric properties were changed between the two renders, yet something is "off" about the lamp lighting. (It might have been a mistake on my side)
If you do not have access to a dedicated rendering/visualization program and you can only render from within Revit, if your renders do not look "perfect", there is no need to start moaning about the "limitations" of the program. You have 5 main options to choose from:
1. Choose which render engine will give you the best result: NVIDIA Mental Ray, or Autodesk Raytracer.
2. If one of the above options do not give you the results you want, how about rendering through the Cloud?
3. If neither one of the above options work for you, start post processing the image inside of Revit. Change the Highlights, Saturation, Mid Tones, etc. to make your image as close to perfect as can be.
4. You always have the option to export your Revit model to an external software program, such as Autodesk 3ds Max, Autodesk Showcase, Autodesk Navisworks, even Autodesk AutoCAD. From within these programs, you will be able to tweak your renders even further
5. Use post-processing software such as Adobe Photoshop.
Make due with what you have. Make what you have work for you.