Canon and Nikon cameras are of similar quality when compared at similar price points and formats, but have some differences for advanced photographers in their settings and lens compatibility. For most consumers trying to choose, it is more important to evaluate which camera you are comfortable holding and operating, and which specific camera meets your needs in terms of shutter lag, resolution, video recording, rechargeable vs. AA-battery powered and other such daily use criteria.
According to a study by PC World, Canon cameras are considered particularly durable, with few problems occurring. Nikon’s are considered less durable, with an average reliability comparable to other digital camera brands. 
A few tips on buying a DSLR camera:
Canon’s cameras and lenses produced since 1987 (known as EOS) are incompatible with those produced before 1987, but are all compatible with other cameras and lens produced since 1987 (with the notable exception of EF-S mount lenses, which are only compatible with Canon bodies with the APS-C cropped sensor) Canon APS-C bodies can use standard EF mount as well as EF-S mount lenses, but Canon full frame sensor bodies cannot use EF-S mount lenses. Canon cameras can also use Nikon lenses but this requires the use of an adapter.
Most Nikon SLR cameras and lenses are compatible with one another, especially those from the same window of 10-20 years. Nikon cameras cannot use Canon lenses.
Older Canon cameras do not have consistent flash performance, with exposure varying from frame to frame. However, the new Canon 5D mark III gives perfect flash exposure. Flash sync options are more difficult to set than on Nikons, as is manual flash mode.
Nikons all have perfect flash exposure, and more flash sync options that are easy to set. 
edit Total-camera-state Recall
Most Canon cameras come with “C” modes, which record everything about a camera’s settings for future use.
Except for the Nikon D7000 and Nikon D600, Nikons do not have total-camera-state recall functions. They have “settings banks,” but they do not recall everything and cannot be locked.
edit Playback Data
Canon cameras show information on a photograph clearly, but will not show the millimeter setting of the lens. Canon cannot show a color YGRB histogram while zoomed in on a small area of the image.
Nikon offers all data on a photograph, but on several different screens. It allows the user to zoom into a small area and see that section’s color YRGB histogram.
edit Lens diaphragms
Canon used 6 or 8 bladed diaphragms up until 2012. These create inferior sunstars and more out-of-focus highlight blobs.
Nikon uses 7 or 9 bladed lens diaphragms, which create superior sunstars and less disruptive shapes.
edit Auto ISO
While both Canon and Nikon allow the user to select a shutter speed to increase the ISO based on lens focal length, only the Nikon allows the user to do so by a fixed amount.
edit Color and Tone
The two brands define colors and tones different, and use a different Auto White Balance. This means that pictures will always look slightly differently on one from the other.
edit Lens Corrections
Most Canon and Nikon DSLRs offer electronic lens correction for dark corners, lateral color fringes and distortion. Canons cannot correct distortion in-camera as shot, while Nikon cameras correct images as they are shot.
edit LCD Quality
Canon’s LCDs have the same 3:2 aspect ratio as the images, while Nikon use a different aspect ratio. Canon cameras have anti-reflection glass or plastic over their LCDs, while Nikons do not.
Since 2012, the two brands have had similar autofocus ability. Canon autofocus is a little faster with cheap lenses, and similar to Nikon with expensive ones.
Canon DSLRs and point and shoots do not have viewfinder grids, while digital Nikons mostly do.
edit Data Embedding
Since 2012, Canons allow for data embedding in the camera, while previous models required the user to add the data using a computer.
Nikons allow the user to embed copyright, name and phone number into every photo.
edit Data Transfer
Photos on a Canon camera can only be read by a computer after installing special software.
Nikon photographs can be moved onto a computer without additional software.
edit JPG Quality
Canon JPG file sizes vary to allow for constant quality, depending on the detail in the photograph. Nikons do not do this, which can lower quality of some images.
edit Award Winning Products
Canon cameras have won many awards, including 5 Technical Image Press Association (TIPA) awards in 2012. These were for Best Professional DSLR Lens (Canon EF 8-15mm f/4L USM fisheye lens), Best Video Digital SLR (Canon EOS 5D Mark III), Best Professional Videocamera (Canon EOS C300 Digital Cinema Camera), Best Digital SLR Professional (Canon EOS-1D X), and Best Expert Compact Camera (Canon PowerShot G1 X).
Nikon won 2 TIPAs in 2012, including Best Digital SLR Entry Level (Nikon D5100) and Best Digitial SLR Expert (Nikon D800).
edit Market Shares and Sales
In 2011, Canon had revenue of 3.557 trillion yen, with an operating income of 378.071 billion.
Nikon had revenue of 887.5 billion yen, with an operating income of 54.1 billion.
edit Where to buy
Amazon.com is always a great place to buy cameras. Here are some relevant links:
- DSLR camera best sellers on Amazon
- Best selling camcorders on Amazon
- Best selling point-and-shoot cameras on Amazon