iv'e stuck with canon for the simple reason i had canon SLR's before switching to digital SLR's i could still use the kit i already owned

iv'e no real preferences either way a couple of friends of mine own nikon D3's with some very nice lenses and they are something else when used in low light situations
i think canon have caught up now on that front with the 1DX and to some extent the 5D3 but it's all bollocks at the end of the day on average more or less all of the current DSLR's no matter what make will produce some stunning results the glass you put in front of the camera is more important than the body anyway and both makes have their own share of quality lenses

granted the pro bodies get things right a lot more than your average consumer DSLR but non the less iv'e seen stunning images from non pro kit the make doesn't matter