In one way or another what I'll say here has probably been said before. To make up for that it's long, so feel free to skip to the next post.
I've used Canon cameras since 1978, starting with the A-1 and F1-n. When the EOS system came along and obsoleted all of my FD lenses I was incredibly upset and really wanted to buy a Nikon system because of that. Many did. When I tried out a Nikon F4 and a Canon EOS-1, the Canon was for me ergonomically much nicer and more intuitive to use. In switching back and forth between the two once I knew how to use both, the Nikon felt awkward. Not wanting to punish myself because I was mad at Canon, I ended up purchasing the EOS-1 and a pile of EF lenses. Since then I've owned most of the 1-series film and digital cameras that Canon has made. They have been bullet proof and the couple times service was needed it was fast and flawless. These cameras have done everything I have needed them to do, sometimes in conditions a camera shouldn't be exposed to. This may sound like Canon "fanboy" talk but it's the simple truth. I'm sure there are Nikon users who'd say similar things.
Unfortunately Canon's present cameras like the 1DX and 5D3 offer vast improvements in terms of auto-focus, auto-exposure, LCD monitor, live view, video, and nearly everything else, except low ISO image quality. If you look at the DXO scores, or your old images if you have had the previous cameras, you'll see that at ISO 100 - 200 the image quality of Canon's best cameras is nearly unchanged since the introduction of the 1DS3 in 2007. That's upsetting, not because the image quality is bad, but because better exists, and like most I want to make the best images possible.
But as others have mentioned there are many shades of gray in the Canon v. Nikon debate, and the "best" system really depends on exactly what you're doing with it. In some situations a lower frame rate can mean lots of missed shots. If you need a light 70-200mm zoom that you'll use under adverse conditions, you'll notice Nikon's F/4 version lacks sealing. The Canon 24-70 F/2.8 is considerably better than the Nikon equivalent. A camera like the D810 using the Nikon lens still beats the IQ of a Canon camera using the better Canon lens, but by less than it ought to. See
http://www.lensrentals.com/blog/2013/01/a-24-70mm-system-comparison. For ultimate image quality there are primes like those from Zeiss, but those aren't weather sealed either, and the practicality of dragging around a half dozen or more primes on top of a couple long lenses to 600mm is a non-starter if you need the ability to travel with all of this stuff. You get the idea. There is no single ideal camera or system.
It would be very odd to lose a client or a sale only because you used a Canon instead of a Nikon, or nearly anything else for that matter. An image buyer's deciding criteria is far more likely to be subject matter, artistic qualities, or the color of their couch than pixel quality. Without side by side comparisons of the same image taken with both systems under identical circumstances how could anyone judge? Heck, I still sell prints of a few images that were shot in the early 1980s using a Canon F1-N, a zoom lens, and Kodachrome 25. They hang next to prints taken with a 1DS3 and 1DX, but people still like them. I have a friend who does well with beautiful pinhole camera images, and another who has sold more than I'd ever hope to, who has often deliberately cranked up the ISO on a digital camera to "bring out that beautiful noise". Regardless, IQ is meaningless if you can't capture the shot you want or need, and you are probably the only person who will ever care about or notice IQ differences in your images that are due to the camera used. There's really a lot more to photography than IQ.
Still, Canon's continuing willingness to let low ISO image quality stagnate in favor of developing everything else under the sun
bugs the living hell out of me! Of course Canon isn't developing a camera for me, they're doing what they think will provide the biggest return on their investments. The only real solution for anyone doing a wide range of different kinds of photography would seem to be owning multiple systems and handling the more complex logistics, or compromising on some aspects of some types of photography.
