why VRay ? why not Mental Ray?

Well you can actually write a thesis on this so I'll try to sum it up in a few points from my humble point of view, for starters Vray has the Virtual frame buffer which is much more user friendly if you need to make any tweaks (color correction, levels,etc..) and memory friendly plus you get all your render channels in the same frame while if you try to render channels (for post production or so) in Max frame buffer you would get a separate frame for each channel which is kind of a hassle..
Vray Displacement is much memory friendly than Max's and Vray Proxies is a huge memory saver specially in large scenes since most of us have hardware limitations when it comes to RAM..
you can never have too much RAM Another thing is the VRay physical camera which gives you much more control of your ..
well..
camera, ISO,F-Stop,Aperture,Exposure and white balance are some of the few things which you can have much more control over with that.
Vray is also compatible with most of the popular 3rd party plug-ins, some are even made FOR Vray while Mental ray falls short on plug-in compatibility, specially when it comes to particles Rendering Motion blur is also much faster in Vray (without going into details) Another great feature is VRIMG and the raw image files VRay creates, you won't know how great that is unless you need to render a huge scene in a huge resolution on a 32-bit system with limited RAM, VRay enables you to do that while Mental Ray can't.
On the other hand Vray may strike new users as cluttered, unorganized and harder to learn and use than Mental Ray but with the amount of documentation, tutorials and forums online you will find more than enough resources to master VRay, however the learning curve might be a little steep.
Vray costs money while mental ray is free, mental ray is much easier to learn and has more memory problems.
this is some of the pros and cons off the top of my head, if that's not enough reason i'd recommend you Google a comparison and see for yourself and judge

vray have realistic matrials and lighting effect then mental ray although you have nearly realistic with mental ray but vray is best for realistic render

I totally agree with Muhammad Fateh... in very simple answer ... both are equally good but you can acheive best results quickly with fewer clicks in vray ... but in case of mental ray they whole lingo is very scientific ... other than that the one reason which really shifted me toward vray was that the native support of mental ray in maya required lots of digging in while vray was providing best things out of the box... i didn't have to find hidden menus, create crazy link for exposure control and so on. 

 

 

both vray and mental ray are good but i choose vray because vray is more popular than mantal ray  " you can find easely tutorials ,material librerie and object librerie in the internet but for mentel ray its very difficult to achieve a realistic render "you have to be a pro " but vray is a littel easy in the bigining :)

Rvery thing is wall but my esey plagings is vray so i used it

Well both are good and can give you realistic image but VRay is quicker to setup for high quality rendering and easier to create materials and realistic lights.
On the other side Mental ray is more powerful because its already comes with "the max" but for very skilled people only, beside creating materials is very complicated and more work to do.

i have some answer for you, i hope u like it :) 1.
more realistic vray than mental ray 2.
if the rendering setting right, vray more fast than mental ray 3.
i think hard to get realistic light in mental ray rather than vray 4.
can use more realistic and high resolution HDRI 5.
the important maybe, more online tutorial about vray than mental ray ^^