Whether it matters depends on whether it makes a difference to the sort of photography you do. I own a 5DII and, within the past couple of months I've rented the 5DIII, 6D and D600 (the latter two simultaneously). I haven't performed any properly controlled tests - certainly nothing compared to what someone conducting a serious review involving test charts, etc. - but merely used them as I would normally use a camera (except that with the 6D and D600 I kept switching back and forth, photographing the same thing from the same place at the same time). While the D600 made good photos, not one ever struck me as superior to those I took with the 6D in any way; at most the differences were fairly small/trivial, and where I had a preference it was for the Canon, mainly because I preferred the colors. So other features were decisive - Nikon's absurdly complicated controls, the weird greenish cast to the D600's monitor, its drab viewfinder (so what if it's 100%?), etc. Relatively trivial stuff I would put up with if it made noticeably better photos, but for my purposes it didn't. I will cheerfully concede that others may conclude otherwise.
+100 I doubt many people have actually done this. So what 's your preference 5D3 or 6D after using them?