I personally find Vray much better and regret how much time I wasted with Mental Ray. But it’s for my needs, if you have thousands of cores maybe it’s fine using Mental ray, if the pipeline is built around it. Also, MR is implemented differently in different packages. In Maya it’s implemented the worst, then Max.
Vray is the guy who does the job, and does well. Mental ray is a guy from an oriental country, which uses clever names for everything, uses complex solutions, and you really have to devote yourself to it like a pupil to a Master. You can achieve similar output with both engines mostly, but it will be much harder with MR for majority of people. I mean the learning curve. It doesn’t mean MR is worse, it’s just not as polished to everyday use, and it had to catch up for Vray, copying its materials scheme (and in some instances even surpassing it).
Mental ray has interesting solutions for speeding up renderings, but it also has some problems, and when you see them in the middle of the project, you see you can’t do much about it, and it’s almost never like this with Vray.
For example MR has (had?) terrible artifacts in specular – small dots, which just don’t go away whatever crazy sampling you use. Very slow motion blur… (I hope it was improved, but from what I’ve read it’s so deep in the core you have to rewrite so much it doesn’t make sense).
And there are many small bugs like this with MR. It drives you nuts, when you don’t have a programmer nearby, who could sort it out. And all you can do is hope and wait until a fix is released, and it takes years.
MR community is hard to deal with, people there are mostly programmers and the usual answer is “you can read the manual… it’s very good”. If you are a programmer, then it’s really good. But if you are not, then good luck with it, when you have a problem.
So at first it looks like MR is a great renderer, but indeed for work it’s quite slow to work with, not so comfortable, and takes a lot of time to learn, whereas the results you get are identical to Vray (and I think for 95% of people it will be like this, unless you really want to use some advantages MR has, like production shaders, custom shaders etc).