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To be honest, I use V-Ray on almost all of my render because I've got so used to it. I can create a rendering setup very fast and I'm familiar with the shaders. I used mental ray for some of my projects, nothing posted here on deviantart, when I needed to get realistic renders on small objects, especial objects that had glass components. I find mental ray a better engine for rendering glass.
In the end, it's a matter of how familiar you are with a certain rendering engine. I'm more familiar with V-Ray, and that's why I use it the most.
As for differences, I don't know, there are different ways of getting a scene set up for a realistic render. I find it more easy to do it in v-ray, rather than mental ray. If you ask someone who uses mental ray instead on v-ray, they will probably say the opposite, so again is just a matter of how familiar you are with the engine. After all, both engines can produce great renderings.