Back in the Canon 20D era, when d-slrs had been stuck at 6MP for a while, I bought the then hot new crop-body amateur camera, the Canon EOS 20D. Its image quality was above that of competing Nikons and Fuji d-slrs. A few years later, I bought the then-current EOS 5D, so I could own a full-frame camera. At THAT time, Nikon had NO FF camera,at any price, only the D2x, which I also owned. The 5D did better at higher ISO values than the D2x, AND was full-frame. The D2x was an absolutely FANTASTIC camera body, with the best AF system one could hope for, but it was hobbled by a sensor that got very noisy very fast, and lost dynamic range pretty quickly as ISO levels went up. The Canon 5D on the other hand was a excellent FF sensor in a crap body...slow AF, no peripheral AF points, viewfinder LEDs that were so dim they disappeared when shooting in sunlight, basically a $389 EOS Elan body for the camera, with a really good $2800 sensor inside. I gave Canon a good, fair shake, and bought two bodies, an 18-125 Sigma DC, Canon 50/1.8, Canon 50/1.4, Canon 100/2.8 macro, 135/2.8 Soft Focus,Canon 85/1.8, 135/2-L, 24-105-L, 70/200/2.8 L IS USM, and Sigma 80-400 OS. A pretty decent, two-body Canon system. I gave Canon a good, long trial.
I prefer most everything about the Nikon system's body and control interface and the entire Nikon "ethos". I've recently re-committed to going back to Nikon fully with the D3x, my first new Nikon since 2005. However, I have to say--with the DOF preview button moved to the right hand side, front of the body, and a FUNC button added and placed below the DOF preview button, Canon is now imitating Nikon's exact control placement, and the new 5D Mark III acts a LOT like a pro-Nikon body from 2005-2012. Ergonomically, the new 5D-III is outstanding, yet it still has a lot of Canon DNA that drags it down for me. I still loathe Canon's button and control concepts and the overall "our users are idiots" Canon engineering ethos. I just do not like the way Canon cameras force me to interface with the controls....like for example, the shooting mode button on the 5D-III has a lock on it!!! Good lord-a LOCK on the mode selector? What am I--a toddler??? And...other issues, too numerous to mention, like the alternating shifting of the command functions of the back and rear control wheels!!! With a Canon, it is always a guessing game as to which control will will move the f/stop and which will move the shutter--because the two dials alternate fucntions, DEPENDING on which shooting mode the camera is in...simply ridiculous...