AARON KARNOVSKI, PHOTOGRAPHY MONTHLY:
"The most popular brands to consider in the DSLR market are Nikon, Canon, Sony, Pentax, Olympus and Sigma. All produce entry level and more advanced cameras. There are subtle differences as well as obvious similarities between the various makes. For example Sony's offerings have a fixed, translucent mirror, whereas the rest have the traditional mirror that moves out of the way of the sensor in order to take a photo.
"One thing that has long bugged the hell out of me is the advice that I frequently read on forums. 'Try a few cameras in-store and whatever one feels best in your hand - that's the one to get.' To me, this is the most ill-conceived method of choosing a camera and literally is the last thing you should consider when buying!
"You need to work out what you're looking for from your camera. If the camera is just intended for family snapshots or photographing the kids then pretty much any DSLR will work for this. However, if to be used in a more genre specific manner, like landscapes, sports, wildlife, architecture, etc., you will want to find a camera that works for the type of photography that you currently shoot or intend to."
Read more, here.