Our towers have a huge power supply, but because of their age, getting all that power to the GPUs is a problem. The fastest single card would be a GTX 980ti, but the (2) 970s will render Thea faster.
For more info on the dual GTX 970s in the cMP, see here - If Nvidia released Pascal (GTX 1xxx series) drivers for OSX, the fastest option would be (2) GTX 1070's as the power requirements are even lower. But that may very well never happen, and I for one am tired of all these bad choices Apple keeps making that hurts us professional users. Looks like after 12 years or so, I'm being forced to move back to Windows.