It used to be more expensive until Nvidia dropped the price. They more or less trade blows and each one has a clear advantage in one or two games a piece. (Skyrim AMD, BF3 Nvidia)
AMD cards introduce frame latency which can cause a choppy game appearance despite high FPS. If you Google 7950 frame latency tech report has a full article about it, and an updated article comparing the performance of older 12 drives and newer 13 beta drivers.
Also, Nvidia drivers are superior in stability.