ATi no doubt has better visual quality, and they dominate the single-card segment by a large margin but when multiple cards (7950 inlcluded) are compared, SLi takes the lead. It's no surpise, NVIDIA's multi-card solution is much more logical and appears to be more thought out and sensible.
That being said, I recently went from an X1900GT to a 7900GS, and I've noticed one interesting thing. The 7900GS at stock tends to get the same or only a few less FPS compared to my X1900GT, which was heavily overclocked. In benchmarks, the X1900GT took the lead, but I guess that makes the rumor true. ATi = bench numbers, nV = game FPS.
Nick