AMD vs...
Nvidia
As you scale the graphical settings, the game starts to become GPU limited, so the high end Intel cpus will perform better with the faster Radeon cards. At lower settings, Nvidia has far less of a cpu bottleneck.
This is also true in Battlefield 4 for DX11, but that game has Mantle for AMD users.
I think the same thing also held true in Civilization V, due to its support for DX11 multithreading. I think AMD implemented a one game solution where they will do DX11 multithreading, but just in Civ V. Since there's only about 3 games that even support DX11 multithreading, I suppose they could do that again.