From a photographer...
Hi,
I am a professional photographer, here's my thoughts on this camera/lens thing:
In the past I've owned 3 different SLR brands Minolta, Nikon, Canon. In 2003 I switched to a Canon 5d because at the time I wanted "full frame". If a Nikon full frame had been available, I would still be with Nikon I guess.
Looking at the two big brands, I've noticed that Canon and Nikon often position their gear "between each other". Canon might have a higher Frame-Per-Second rate on one model, where Nikon might have higher resolution, or other features. Better/worse AF-system, bracketing,...
One model up or down (in price reange) it might be vice-versa.
The Canon f4/24-105L IS (for full frame) was, for a very long time unmatched by Nikon. I couldn't live without the 24-105: image stabilized, it's a great walkaround lens for travel. One example: Nice for some candid low-angle shots, at 24mm and 1/4 second from street level, to get blurred traffic / car lights.
Now Nikon has a 24-120 lens. And sometimes I wish Canon had one too
BUT!!!! Just because of that, I won't change systems.
Regarding cameras:
The reason why professionals usually have 2 cameras is: to have a back-up. I just can't afford my camera breaking whilst I'm doing a job *an unhappy customer won't come back.
Or (for journalists): each body is equipped with a different lens. 24-105 on one camera, 70-200 on the other camera. Other than that, you don't need more than one camera.
So: Have a look at what you really need (not what you want) in terms of frames-per-second, weight, megapixels,... and then buy a camera that fits your needs. Then look at what kind of work you want to do & buy 1-2 lenses. Journalistic work or street photography go wide-angle. Wildlife go tele & macro. If you go tele: different lenses have VERY different focusing speeds. For example, Canon's 2.8/70-200 is way faster than their consumer f4-5.6/70-300.
Then stick with the system & buy new glass whenever you can afford it. I know photographers who shoot with very old lenses, but buy a new body every 4-5 years (some even more often).