Actually the Nikon is probably the sharper of the 2, especially over the central frame area:
http://www.lenstip.com/358.4-Lens_review-Canon_EF_24-70_mm_f_2.8L_II_USM_Image_resolution.html
http://www.lenstip.com/298.4-Lens_review-Nikon_Nikkor_AF-S_24-70_mm_f_2.8G_ED_Image_resolution.html
Canon center wide open is at the 35-39LPMM level, Nikon is at the 39-47LPMM level
FF edge, Canon is at 25-28LPMM, Nikon is at 27-28LPMM at 24 &45mm, but 23LPMM at 70mm, stopping down to f4/0 the canon is at 38-43LPMM, the Nikon at 39-43LPM.
So both almost identical at frame edge, the Nikon is sharper in the center, quite significantly so wide open at all focal lengths.
I suppose we can keep quoting tests over and over...
http://www.dxomark.com/index.php/Pu...-2.8L-II-USM-A-Peerless-Performer/Comparisons (look at the bottom).
The fact is real world, there probably isn't much in either lens.
That said the 800D sensor does knock anything that Canon presently has into a cocked hat. This new Camera should do the same.
Canon really really really need to step up to the plate with their sensors - they're rapidly falling behind.