Both cameras are excellent digital cameras capable of taking excellent photographs. The differences in photos will have more to do with the eye and technique of the photographer, rather than any minor differences in the cameras. One camera does have an edge over the other, but I will get to that in a moment.
Whatever is the best camera of the moment will change in a few months to a year. The Canon 10D was the best camera in its class at the time of its introduction. When Nikon D70 came out, it was in many ways better than the Canon Digital Rebel and some would say it gives the 10D a run for the money. For me, the Canon 10D is better because the Nikon D70 does NOT have mirror lock up, a feature that I consider essential for a lot of my nature photography. This is a glaring omission on Nikon’s part. Other than that, it is a fine camera. Now the Canon 20D is available and most reviewers say it is better than the Nikon D70.
SO WHAT!! Most folks can’t afford to spend the rest of their lives buying the latest and greatest camera, much less switching lenses.
The more important question is “What lenses do you have or want to get?” Repeat after me: “LENSES ARE FOREVER AND CAMERA BODIES ARE TEMPORARY.” That is an obvious overstatement, but gets at something important. All of my lenses purchased over the last 20 years still work fine today. A set of well chosen lenses usually costs more than the camera body. Even in the good old days when a film camera body was purchased with the expectation of using it 10 or 20 years or more, wise photographers put their money into better glass rather than the very best camera. This is even more true today as digital bodies are regularly replaced by newer and usually improved versions. Choosing a good lens system is the number one priority. Then pick a camera that will use those lenses.
If you have a set of Canon EF lenses that you like and will continue to use, then your decision, if you are switching to digital, is to pick a Canon EOS digital body (or Kodak body) that you can put your Canon lenses on. Depending on cost and features, you can choose between the Canon Digital Rebel, the Canon 10D, the Canon 20D, one of the “pro” series bodies (1D Mk II or 1Ds Mk II), or the Kodak DCS Pro SLR/c. The 20D is so feature rich and produces such high quality images that it is more than adequate for many professional uses. (A photographer for NATIONAL GEOGRAPHIC shot an article for the January 2005 issue using a pair of 10Ds.)
If you have a set of Nikon lenses that you like and want to continue to use, then your decision, if you are switching to digital, is to pick a camera body (Nikon, Fuji, or Kodak) that you can use with them. By all accounts, the Nikon D70 is an excellent choice. If you have a set of Pentax lenses that you like and want to continue to use, then a Pentax *stD is a good choice.
The same applies to other brands of lenses, providing you have a digital body that you can use with your lenses.
If your lenses have been orphaned without a digital body available, then you are faced with starting over. If you want to switch lenses because your brand doesn’t have some specialty lenses that you want, then you have some choices to make. Or maybe you don’t have an SLR body and want to jump to digital, then you have some choices to make.
All of the major camera manufacturers offer good to excellent lenses, with some variations in quality within each of their own lens lines. Reviews can lead you to the better lenses in each brand. Some of the “aftermarket” lenses are quite good too.
I like Canon lenses for several reasons.
1) A good selection of IS (image stabilization) lenses. They allow me to hand hold photos at much slower shutter speeds than would otherwise be possible. Other manufacturers have lagged behind in this area.
2) Tilt-shift lenses in three focal lengths. Tilt-shift lenses allow me to tilt to extend depth-of-field, to shift for perspective control, to shift for creating easy to stitch high resolution panoramas, and to tilt and shift to play with perspective in creative ways.
3) A nice collection of macro lenses and accessory flash units, including the specialized 65mm 1X-5x macro lens and the MR-14EX Ring Lite.
Your needs may be less specialized than mine and these feature may not be as important to you. If you don’t need specialized lenses, almost any brand and a carefully selected set of lenses will do.
If you are switching to digital, your primary decison relates to lenses, your secondary decison relates to camera bodies.
When the latest Nikon comes out should Canon owners despair? No. Should Nikon owners fall into a funk when the latest Canon comes out? Of course not. Camera envy is not good for your photography. You should spend more time thinking about your next picture than your next camera body. If you think more about equipment than photographs, you are a collector (at least inside your head), not a photographer. If you chose a good camera 6 months or a year ago, it is still a good camera. If an improved model has features you want and won’t break the bank, fine, get one. But don’t spend too much time obsessing about equipment. Spend time taking pictures.
So which camera is better the Nikon D70 or the Canon 20D? I only own one Nikon SLR body, and it is pre-autofocus, so I am not qualified to say. I can tell you that most respected reviewers that have used both would give the nod to the Canon 20D as the better camera. DP Review, one of my two favorite review sites, calls it the best in its class. As I mentioned before, the Nikon D70 does not have mirror lock up, a serious omission when it comes to closeup photography and long lens work. Both the Canon 10D and 20D have mirror lock up.