You can probably get a good read on the Video card GPU comparison by comparing the benchmarks in the Holomark thread: Holomark 2 Released!
Vray is a little harder to determine. I expect being a raytracer that the floating point speed of the cores times speed of the cores times the number of cores will give you a general benchmark. Raytracers tend to be very CPU intensive. You could take a look here are CPU scores. http://cpu.userbenchmark.com/ . That site is great for general machine speed. CPU benchmark should reflect Vray speeds, but the GPU scores on the benchmark site are not accurate to Rhino. Holomark is better for that.
You can ask if anyone actually has run Vray on both chipsets on the Vray thread: https://discourse.mcneel.com/c/rendering/v-ray
I hope this helps.