I get asked sometimes what brand of equipment I shoot with. The short answer is, Nikon, though I've shot both extensively. But it doesn't matter what pros shoot with when you're deciding what kind of camera and lens to get. So what matters when choosing a brand? What your friends shoot. Really. It's that simple. What's more complicated is which model is best for you, but that's for a future post.
Despite all the talk about equipment, what's most important is the photographer. You'll shoot pretty much the same with either Canon or Nikon; how good your photos look won't depend on your equipment, but that's for another post. Don't make the mistake of seeing an amazing photograph, then wonder what equipment was used and assume that the equipment made the shot.
Canon and Nikon only
The idea is straightforward: Canon and Nikon are the world-leaders in the SLR category; neither stands out from the other. So my advice is, stay within these two brands; while Sony is catching up, they're just not there yet. Nikon and Canon have such a deep array of bodies and compatible lens systems reaching back decades, staying within these two brands is your best bet. Lenses are often more important than the camera body you choose, so sticking with the companies with the deepest background in lenses is important. (And if you choose just one lens, and you like shooting people, get a 50mm fixed/prime lens with a f/1.4 or f/1.8 aperture. On the Canon side, the f/1.8 50mm is only $100 and will give you more satisfaction than zoom lenses 10 times the price.)
What do I Shoot and Why?
I shot Nikon all my life until 4 years ago when I switched to Canon. At the time, the Canon 5D had come out and was the only "full-frame" sensor camera out there at a reasonable price, and Nikon just couldn't compete. Full-frame vs Crop-Sensor (Canon 5D vs Canon Rebel, Nikon D700 vs Nikon D90) is another discussion entirely, but suffice it to say, I wanted to not ever worry about lens-compatibility, so I went full-frame. [discussion and explanation here] But then this year, after the Canon 5D Mark II arrived, I was disappointed with its lack of auto-ISO features. So I went with the full-frame Nikon D700 and never looked back. It's an incredible camera that beats the 5D Mark II in areas that are important to me, though the 5D's video is amazing and is not found on my Nikon D700 (the Nikon D90 has HD Video).