Throughout all of our testing it was apparent that the NVIDIA GPU based video cards were providing better performance and more enjoyable gameplay than the AMD GPU based video cards. There were no instances that we found where any AMD card provided better gameplay performance than it's NVIDIA counterpart. We also experienced an issue which we believe to be driver related with all AMD video cards. We experienced an extremely noticeable lag any time there was a large amount of debris flying around, explosions from missiles, grenades or vehicles, or buildings in the environment being destroyed. This was not present on any of the NV based GPUs.
The NVIDIA GeForce GTX 580 SLI and AMD Radeon HD 6970 CFX were both able to play Battlefield 3 Multiplayer at 2560x1600 with FXAA High, 16X AF, and all Ultra in-game graphics options with HBAO selected. They performed 12-16 average FPS above our 60 average FPS goal with maxed settings and were both enjoyable. In the Apples-to-Apples tests the AMD Radeon HD 6970 CFX had a 1 FPS advantage but the lag during explosions was much more evident than during highest playable settings testing, thus providing a worse gameplay experience.
The NVIDIA GeForce GTX 580 and AMD Radeon HD 6970 both performed within .2 FPS of each other at 1920x1200 with 16X AF, FXAA High and all Ultra in-game settings including HBAO. The NVIDIA GeForce GTX 580's performance was favorable without the lag that all AMD cards experienced.
It's been said before, but if possible just stick to one high end card. Running 2 cards in scaling (CFX or SLI) you will almost always have problems. You will also see some micro-stuttering and run into driver issues or games that don't support it well enough to justify the costs. Having tried both SLI and CFX I can say that personally SLI was a better experience and more games are optimized for that architecture (if you must go for multiple cards.) Just my 2 cents.