While mental ray is trying desparately to catch up with V-Ray, there are still a number of things that V-Ray does better:
(*) V-Ray has physically accurate lights and materials (the arch&design and ProMaterials based on it are not quite so, neither are the mr area lights, including the light portal); of course, any physical accuracy only goes to a certain extent, but in V-Ray things are a lot closer to what you’d expect;
(*) V-Ray has very efficient image-based lighting tools - the texture-mapped dome light, as well as texture-mapped rectangle lights. This will be extended with true object lights in SP3. There are no equivalents of these features for mental ray.
(*) V-Ray handles a lot of lights better;
(*) V-Ray offers much easier GI setup with the light cache;
(*) GI in V-Ray is typically with much better accuracy and detail for equivalent render time compared to mental ray (where you are practically forced to use ambient occlusion to get the same kind of detail);
(*) V-Ray offers more and efficient methods for handling animations (both fly-through and moving objects);
(*) The V-Ray shading pipeline supports efficient layering of materials (e.g. with the VRayBlendMtl material) - equivalent materials in mental ray typically render a lot slower; V-Ray also supports faster and more accurate sampling of reflections/hilights from area light sources and IBL.
(*) VRayProxy is still better than the mental ray implementation in terms of handling larger amounts of geometry and handles motion blur for deforming proxies more accurately;
(*) V-Ray has better displacement with more options (e.g. 2d landscape option);
(*) V-Ray still has faster raytraced motion blur.
It is also worth noting the fact that it is mental ray that takes features from V-Ray, not the other way round
For example, final gather in mental ray went through several revisions in the recent years, before finally landing on a solution that is remarkably similar to the irradiance map used originally by V-Ray (of course, this particular fact can be said about the other two 3ds Max plugin renderers :-)); light portals and proxy objects were originally introduced in 3ds Max by V-Ray etc.
At the same, V-Ray also moves forward and there are many new additions planned to make your lives easier…
Best regards,
Vlado