Someone shooting both brands would be best to answer you obviously, but as a Nikon shooter knowing many Canon shooters, I don't have the feeling that Canon high end bodies perform less well than Nikon's in this area.
At higher ISO, Nikon's bodies tend to apply less noise reduction than Canon's, which leaves more noise in the images, but is seen by some people as producing a more natural "film" like result. Overall though, Canon bodies are said to have the upper hand at high ISO still.
Regards,
Bernard