I ve tried both. Vray works better for me.
-Gpu render (for setting lights works)
-multi tread support @ render
-if you start a huge render like 30-40minutes one. It wont freeze your maya... In brakets you ll see how its calculating

I remember earlier i tried render cars with mentalray.. I did not know what is happening. It completly froze my maya..... after 45min the render was ready.