Neither Nikon D70 (upgraded version is called D70's' which is 99% the same except the bigger LCD) nor Canon 20D is better on absolute terms. Each one is better than the other depending on what you want to use for.Both cameras are superb. Lets say:
Fast Action Shots: For fast action shots and for sports photography 20D is the winner because it has almost twice the frame rate than that of D70s.
Low light situations: 20D is the winner because you can crank up the ISO setting up to ISO 3200. D70s can go up to ISO 1600 meaning 20D can get twice the brightness in its pictures if you were to shoot with D70s in the axactly the same low light situation with its highest ISO setting on.
Price, Image quality and over all ease of use: Almost the same while D70 is cheaper, smaller and lighter than 20D.
Portibility: Because D70 is smaller and lighter and easier to adjust quicker than 20D, D70s will be the preffered one therefore making it ideal for travel photography and for the use in the field. This is reason why many Pro-photographer also use D70 on dalily basis besides their heavy and bulky Nikon Pro-cameras like Nikon D2H and Nikon D2X.
Adjustibilty: Because lighting sitution never remains the same in the real world, D70s is far more easier to adjust to suit the ever changing lighting situation in the field.
Image quality: D70 comes with 6 MP while 20D with 8 MP. The difference is simply too close to see by your eyes. You will see significant difference only if you make prints that are bigger than 14 inch across in size.
Color sautartion: If you are a NOT a photo nerd dont worry about it but for me it matters and I see Nikon D70 is the possible winner because there are two places where I can jack up the color saturation as opposed to just one in 20D.
Flash sync: A strong advantage for D70 because it has flash sync speed of 1/500 as opposed to just 1/250 on 20D meaning the flash on D70 recycles faster so that you dont have to wait for the flash to recycle before you can take the next shot. This feature is important if you are using a SLR for event-photography sush as parties, ceremonies, dassian, wedding, pasni etc where you will be forced to use flash because of insufficient available(ambient) light indoors.
Image Sharpness: D70s is little sharp over all however you can to tweak the camera settings to either jack up or crank down the shapness.
WB or White Balance: Each people have their own taste so you can tweak the settings in both cameras to get your taste.
Menus: D70 is a lot easier to get around menus and change camera settings after all the lighting conditions always change in the real world so this the menus have to be easily accessable without fussing and getting lost a lot to be able to quicly change the camera setting.
ISO Setting: Canon has advantage becuase it can go to ISO 100 up to ISO 3200 while D70 can go from ISO 200-1600. I would have loved to see D70 offer ISO 100 because I love to see great color saturation on my pics.
Noise or Digital Artifacts: D70 shows more noise at higher ISO because it doesnt apply noise suppression filter as mush as it does in 20D. Therefore D70 images are noiser but sharper while 20D images are less noisierbut less sharper. In my experience, noise level on my D70s at ISO 800 looked ok for me.
Contrast: D70 offers more flexibility to move around and set the best possible contrast level based on lighting of a subject while 20D is less flexible.
Image quality setting: It's based on your preference. I prefer D70 because when writing with JPEG file because it offers file sizes of 3 image quailities while 20D offer only 2.
Battery life: almost equal. You can make about 1000 pics in average from a freshly recharged battery on D70.
Folders and file numbering: D70 is the winner because it's more systematic and chances you loose your image files and folders in your computer is far less. D70 never repeats a filename in the entire life of the camera.
Media and memory: Both use CF cards
Lenses: Equal because both Canon and Nikon has a huge line up of spectacular lenses and both cameras can use third party lenses like Sigma and Tamron with no glitch at all. Personally I dont prefer third party lenses.