NVidia cards do not efficiently implement bit rotation, while AMD cards do, and it happens to be the core part of the SHA algorithm used for bitcoin. In general for an arbitrary task they're fairly close in performance.
AMD cards dominate Nvidia in pretty much all password hash bruteforcing algos, even those that do not rely on bit rotation (bit_align). See
http://golubev.com/gpuest.htm for example. It is true that another instruction helps in more cases (bfi_int which I talked about at http://blog.zorinaq.com/?e=43) but in general, AMD cards have a lot more raw integer and floating point compute resources (execution units) than Nvidia cards.
