Generally speaking, the 7950 wins hands down, but since it seems you are interested in 3D, the answer is a bit more muddled.
The answer probably comes down to how much you think you are going to use the 3D. If you think that you will use 3D a lot (more than 2D), I would lean towards the 580. Otherwise, go for the 7950. Do be aware that if you go for the 580, you are choosing a slightly slower card that uses more power and is from an older generation.