To be hionest not many Maya Artists can answer that question as VRay is only on Beta at present for Maya, so doubtful that anyone has used it.

Max users however will be able to answer that as they both have the ability to user MR and VRay.

From what I have seen VRay creates nice renders, but so does MR. Having used MR, its a bit of a beast to get your renders 100% right with a lot of tooing and frowing to ammend settings. TBH you would have to try eboth and make your own descision. I would go for MR as its great, although its a bit fiddly.