Some may not agree with me on this but it seems to me that Nikon has moved ahead of Canon in the sensor department in the last 2-3 years, with up to 2 stops more Dynamic Range at base ISO. Rightly or wrongly, I believe this has been a big selling point.
But with the recent release of the Canon 1Dx mkII, it appears that Canon may have bridged the gap in DR that I mention above. The word is that they have finally integrated their analogue to digital conversion stage onto the sensor chip, just like Nikon/Sony, thus improving the read noise performance significantly. The 1D x is probably out of reach of many financially but things will get interesting if Canon start using similar sensor technology on camera models below top of the range. I think they will but we'll have to wait and see. For now Canonites, hang on to your L glass.
Dave