AMD Crossfire performance at 2560:http://www.tomshardware.com/reviews/battlefield-3-graphics-performance,3063-11.html
NVidia SLI Performance at 2560:http://www.tomshardware.com/reviews/battlefield-3-graphics-performance,3063-6.html
6950s (2GB) in crossfire slightly outperformed (on average, in this benchie) the more expensive 570s in SLI. Either setup will get you where you need to be.
There's nothing wrong with brand preference when you have two competitors putting out really solid products. 6950 crossfire (and most likely 560ti SLI) and better will be great. Factor in nvidias generally higher power use and AMDs general reputation for more finicky drivers. You really can't make a bad decision these days as long as you're shopping for a good deal on the model you decide on.