The results are more like a mixed bag, depending on the specific scenes. In some cases the GPUs perform better, in others - the CPUs.
Our CPU server configuration is Dual Intel Xeon E5-2680, with 16 cores and 32 threads per machine.
Because of the way the numbers vary from scene to scene, I would encourage you to sign up and render a few frames from your project, to see the render times for your specific case.