Both are very powerful render engines. Iím not saying Mental Ray is slower. It depends on the user. Iím using both with 3Ds Max. For me Vray can make beautiful images but Mental Ray physically more accurate than Vray. And you can push easily higher render quality and details than Vray. BRDF tuning also plus point in Mental Ray. This is just my idea.
Then again depend on the user... Thatís what makes us different. Not the tool. As all said and I also agree without Vray you can survive but Without Photoshop you canít.
Most of people still think Vray is the only render engine which can make realistic renders. That is why all are asking ďWe are looking for a 3D Visualiser, excellent in 3Ds Max and VrayĒ. I see no point in that.