Nikon vs Canon- what's the difference?

Is it simply preference?
What would be the best DSLR for someone pursuing photography as a side job?

I would say simply preference just like Apple and Android. I would say the model of the camera it what makes the difference. I have a Canon 60d, i'm a beginner and that's a really good camera to start off with. In this website >>> http://www.cnet.com/...w.cnet.com <<< you can see reviews of different cameras, see what fits u best.

Mostly it is just preference, unless you are after some specific feature that is better presented by one or the other. I personally use Canon because I rather like their lens range, but that's not to say that Nikon has it worse.

Check dpreview.com for excellent reviews.

None whatsoever. Olympus are smaller and lighter, Sony do better video and I'd take Pentax over any of them.

There are differences between the two and it all depends on which body you are considering and what you shoot. Nikon in general has better low light capabilities and also is higher performing on the pro cameras. You can't go wrong either way.