Taking the middle resolution of 2560 x 1440, we watched the CPU utilization. Across the different resolutions we tested Dirt Rally at, the CPU utilization was similar.
Sapphire Nitro 390
For both video cards, the utilization was very similar. They were both hovering in the mid-40% range. In an attempt to get a similar CPU utilization, the sample utilization was grabbed at the 1.5 minute mark within the 3 minute benchmark. At that specific time, the Sapphire Nitro 390 had a CPU utilization of 44%, while the eVGA GTX 970 was slightly higher at 48%. Neither of these would raise any alarms, as they are still below 50% utilization.
Some developers optimize their games for a specific GPU chipset, while others tend to leave it open. Codemasters Dirt Rally has been optimized towards the AMD Radeon chipset, and the optimizations that they have done, show exactly what is possible. In a game like Dirt, you are moving so fast that the majority of the background gets motion blurred, which would allow you turn down the graphics to get the framerate you desired. However for at higher resolutions, max details and the smoothest framerate, an AMD Radeon 390 GPU would be my recommendation for Dirt Rally.