Performance in Simulators
The GeForce GTX 580 SLI enjoys but a very small advantage in this game: 5 to 14% at the first two resolutions. Then, at a resolution of 2560x1600, the Radeon HD 6990 goes ahead. Fortunately, Nvidia's solution delivers a playable frame rate in every mode.
Performance in Strategies
StarCraft II: Wings of Liberty
Nvidia's cards have problems working together in SLI mode. Despite the higher clock rates, the SLI tandem is slower than the single GeForce GTX 590. The frame rate is playable even at 2560x1600 but investing into this graphics subsystem for playing StarCraft II doesn't make sense, especially as the Radeon HD 6990 offers you more speed.
Total War: Shogun 2
This game makes you choose between FSAA and a full selection of special effects. We prefer the latter.
The GeForce GTX 580 SLI is most appropriate for playing Shogun 2, especially if you want to do that on an Ultra-HD monitor (2560x1600 or 2560x1440). The GeForce GTX 590 delivers good performance in the Ultra-HD mode too, yet the SLI tandem is about 25% faster!