Its all about preference at the end of the day and the differences in image quality varies from model to model. I used to lead up a team of photographers covering a music event, they were about 50/50 Nikon and Canon (note nothing else!), most of these guys were semi-pro rather than pro.
A lot of the photography was low light stuff (think concerts), so high ISO, the first few years you could spot the Nikon shots apart from the Canons due to all the extra noise (this was about 6-7 years ago now), but over the years this changed. In the end due to peoples different post processing etc, I found it impossible to tell what brand images were taken on, even upon close inspection of the images. The organisaion we were shooting for was happy with all of them.
Personally I shoot Canon, but that is because I bought one in 1995 (EOS50) because it had dials instead of those darn buttons that everyone else had at the time which in my opinion that made it more user friendly, now every thing is buttons again! I am staying Canon as in the last 15 years or so of using them I have amass quite a bit of gear that will not work with anything else.
If I pick up and try and use a Nikon it frustrates the &*#(&* out of me, much the same way as my Nikon mates do when they pick up my Canon!
These religious brand wars are pointless, Canon is there to keep Nikon on their toes and Nikon is there to keep Canon on theirs, the end result better cameras for all of us