Good run-down of the Canon Digital Rebel and Nikon D70s in this article by Philip Greenspun.
Digital single-lens reflex (SLR) cameras are the standard tool for serious photographers. With the recent introduction of cameras such as the Nikon D80 and the Canon Digital Rebel XTi the market for digital SLR cameras has expanded tremendously. A point-and-shoot compact digital camera can offer reasonably good image quality but a digital SLR, which usually looks a lot like an old standard 35mm film camera and may use the same lenses, offers the following advantages:
* accurate, large, and bright optical viewfinder
* fast operation and large controls
* excellent image quality in low “available” light situations when it is necessary to use higher ISO speeds
* interchangeable lenses