VRay is the darling of Architectural rendering with a real dominance of the 3ds Max visualisation in this area. That said 3ds Max ships with Mental Ray and soon enough we will have MR in Revit as well. We have been looking at the VRay plugin for Sketchup as well, so how do these renderers compare...
The generically accepted position is that Mental Ray is powerful and flexible, but comes with a mighty steep (and long) learning curve. This means that people can dive into VRay and get beautiful results far more quickly and easily - this is a real asset for students and this pattern seems to hold true in FBE. Obviously MR comes along with the ride with Max, which should really count for alot - but VRay is just loved by this industry - and that is for a reason.
What does the web and the vast blogosphere have to say and show (ignoring all those comparisons from eons ago). Well the consensus is definitively that MR is hard work in comparison, but more powerful in the long run - sound familiar... Indeed the number of seriously happy VRay people out there is notable - definately not the same joy for MR except perhaps when I see people talking about the lastest (2008) releases. I almost get the impression that if people can get nice results from MR, then that is a real achievement, whereas people just expect it from VRay. Oh and there are the fans of other renderers like Maxwell out there as well.
Threads of note (hit Google for more):
Autodesk Forum - battle over GI
CGArchitect handling large poly counts
Simply CG MR examples doing the VRay thing
Unfortunately I failed to find a real comparison of the two renderers and what they can do side by side in terms of complexity, time and quality - esp with the current versions. (We have to do some!)
The only exception was this neato threat using Max9 by Noe2 on Evermotion (aka Dian Maulidi). Below are first the VRay versions and then the Mental Ray ones (pretty similar huh. Perhaps the VRay ones are a touch more refined, but at the same time less punchy - which is the VRay way):
[Noe2's Vray Renders from the Evermotion site]
[Noe2's Mental Ray Renders from the Evermotion site]