It's actually a matter of taste, and even though vray has alot of speed advantages, it's not really that big a deal (ok it might have changed since I used it, please don't flame me) mental ray is also quite a speedking, it's just less artist-firendly, but does the same and comes with 3ds max.
AFAIK there's a metallic car paint shader in vray, making the actual material creation really easy, where you ahve to make it yourself in mental ray.
There are tutorials along with 3ds max for the mental ray system, but be prepared, it might be quite heavy to deal with at first, because of the very misguiding namings used for some settings in mental ray.
To enable mental ray, press F10, and under renderer, find your production renderer, and shift it to mental ray. It should be pretty easy to locate
The rest you will ahve to learn through the tutorials...