Everyone seems to have something to say about this topic, but I have to say something about it, even if I’m cliché. Many people, including some professional photographers, believe that digital image quality is synonymous with final image size measured in megapixels (MP). But this is a gigantic farce. Even 6 MP can be enough for an image printed smaller than a standard 8.5×11 page. Of course, it’s always nice to have the option of going bigger. Thus, most commercial photographers in Minneapolis have either 16 or 22 MP cameras. However, there is a school of the digital back, and there is a the school of the DSLR.
Just for clarification, DSLR stands for digital single lens reflex. It is a camera that most people identify by its ability to change lenses. DSLRs such as the Canon Digital Rebel, are priced so that consumers can purchase them (less than $1000). Other DSLRs, such as the Canon 1DS Mark II, are more common professionally because of their 16.7 MP chip and high price tag ($7,000).
A digital back, on the other hand, is part of a system even more modular than the DSLR system. It consists of only a light sensing chip mounted to an image processing unit. The backs are made to attach to most medium format and large format cameras and are exclusively used by professionals because of their 22-39 MP image sizes and very high cost ($20,000 - $30,000).
Because image size after a certain point is usually irrelevant, here is a comparison of Phase One’s P25 to Canon’s 1DS Mark II excluding the difference in image size:
Phase One P25

16-bit color depth (65,536 shades) per channel
12 f-stop dynamic range
9 micron pixels
48.9 x 36.7 mm CCD size
excellent lenses available (can use any camera system)
50-400 ISO range
35 captures/minute
best to shoot tethered
cumbersome
Canon 1DS Mark II

12-bit color depth (4,096 shades) per channel
10? f-stop dynamic range
8.2 micron pixels
28.7 x 19.1 mm CMOS size
decent (not the best) lenses available
50-3200 ISO range
8.5 captures/second
easier untethered shooting
less cumbersome
Color depth is the total number of colors possible on each channel (red, green, and blue). Dynamic range is the range of light brightness that can be captured. Larger pixel sizes and chip sizes serve to reduce noise in the final image because of the way that photons are interpreted by them. Smaller pixel and chip sizes can also be negatively effected by lens sharpness to a greater degree than larger ones.
And so, while DSLRs can be more useful in many situations, digital backs produce the highest quality image (hands down) because of their greater color depth, wider dynamic range, and larger pixel sizes (and therefore larger chip sizes).
If you found this post to be excruciatingly boring, please excuse my gearheadedness. Apparently, this topic is all I can stand to write more than a few sentences about.