I work at a photo lab, we do not sell cameras, but customers sometimes ask for advice on a camera to buy. What I tell them is that it is almost impossible to buy a bad camera today. The most important factor (in my opinion) is to get one that feels comfortable to you. I mean this both in terms of how it feels in your hand (body design) as well as overall layout of buttons and options.
If we are talking SLR cameras, Canon has a more intuitive and user friendly layout in my opinion. I teach a basic camera class at the lab and sometimes finding the settings on a Nikon camera is a nightmare.
In terms of quality, they are all good. Some prefer Nikon because they say it has a wider dynamic range than Canon. As mentioned by Jackwow, many like Canon because they have the largest selection of lenses available.