The Canon seems to have lost the competition with Nikon according to sensor and processor technology. Good for Canon that at least Lenses are unbeatable.
the main advantage that the Sony Exmor sensor has is about dynamic range.
For High-ISO, we speak of 1/2 stop -according to DXOMark measurements- between the very best Nikon and the 5DIII, which is not really much relevant...
As you say, you may well have more differences in lenses performance.
1/2 stop is not much !
And, to be honest, for landscapes, we used to be able to have good results from less performing sensor with less dynamic range. HDR is one of the ways.
Of course, it doesn't mean more dynamic range is not useful (think of a wedding, where black and white are so important ;-) ).
But everything is relative. And, yes, I think lens quality is more relevant than the sensor differences we see nowadays.
I own an old 1DII and a Nex7 and, to be honest, high ISO noise is a problem for me for both. I need one or two more stops! And while I greatly appreciate the 24Mpixels and the greater dynamic range of the Sony, for some pictures, I still get better results from the 1DII with the same lens.