After suffering through that thread of Best Buy in-store comparison shots of the D70 and 20D I thought I would dispel the mythology of substantial differences between brands.
Are the brands different? Certainly, that's why I changed brands. Is the difference obvious? Absolutely not.
The difference between Nikon and Canon is subtle to the point of indistinguishable when shot conditions are favorable and the output is 800 pixels wide and posted on the net.
The Best Buy "test" proves once again the the biggest impact on image quality is the person gripping the camera.
I don't know how anyone could make a $1000+ decision based on those shots. It's like picking a restaurant based on which one made you less ill. 
Take a look at these 2 shots with 100% crops. These were both processed through ACR 3.1 - the only post processing in PS CS2 was sharpening. WB for the Nikon shot was "custom" using a WhiBal card. WB for Canon was "auto" because I'm too dumb to remember to apply the custom WB I set on the first shot of the shoot (still getting used to Canon custom WB approach - doh!). :roll:
The brand identification for each shot is at the bottom of my message but DON'T LOOK without giving yourself a chance to honestly evaluate both shots.
Set up - 1 Elinchrom 600RX with 3'x4' Chimera Softbox - no fill light or reflectors. Canon 20D with 50 1.4 @ f 11 and Nikon D2H with 50 1.4D @ f10.
Shot 1:
100% CROP:
Shot 2:
100% CROP:
Not so easy to tell them apart is it?
(See below for brand identification . . .)
So what are the difference?
First, Canon does a better job with skin tones out of the box for MY tastes (I'll say it again - MY TASTES - no flaming here).
The Nikon shot still has a slight green hue - even with custom WB. Canon skin tones look very good with my "accidental" auto WB - you would be lucky to get the same skin tones with Nikon on auto WB. This is a minor difference but when processing 200 images Canon is easier for me because the skin is on the money more often.
Canon has less shadow noise - even at low ISO. If you underexpose a Nikon studio shot by 3/4 stop or more at ISO 200 the shadows are a bit messy with color noise. I know you shouldn't underexpose in the studio but this also applies to existing light and we all make mistakes. If you miss exposure with Nikon, even at ISO 200, there is shadow noise which can be troublesome when printing.
Both shots print beautifully at 13x19. BOTH cameras are wonderful and I'd be happy with either one. I went with Canon for very specific portrait reasons (and camera weight which is a big difference). The Nikon 13x19 print is just as sharp as the Canon print with "half" the pixels.
The Canon file is just as sharp as the Nikon file with no in camera sharpening (gasp!). 
So let's put the following mythological creatures to rest at the bottom of the ocean:
Canon is over processed in-camera and soft - bunk.
Nikon has too few pixels to compete with Canon - bunk.
Brand Identification:
Shot 1 - Nikon
Shot 2 - Canon
Hope this is useful.
The difference between Nikon and Canon is subtle to the point of indistinguishable when shot conditions are favorable and the output is 800 pixels wide and posted on the net.
Take a look at these 2 shots with 100% crops. These were both processed through ACR 3.1 - the only post processing in PS CS2 was sharpening. WB for the Nikon shot was "custom" using a WhiBal card. WB for Canon was "auto" because I'm too dumb to remember to apply the custom WB I set on the first shot of the shoot (still getting used to Canon custom WB approach - doh!). :roll:
The brand identification for each shot is at the bottom of my message but DON'T LOOK without giving yourself a chance to honestly evaluate both shots.
Set up - 1 Elinchrom 600RX with 3'x4' Chimera Softbox - no fill light or reflectors. Canon 20D with 50 1.4 @ f 11 and Nikon D2H with 50 1.4D @ f10.
Shot 1:

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100% CROP:

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Shot 2:

Subscribe to see EXIF info for this image (if available)
100% CROP:

Subscribe to see EXIF info for this image (if available)
Not so easy to tell them apart is it?
(See below for brand identification . . .)
So what are the difference?
First, Canon does a better job with skin tones out of the box for MY tastes (I'll say it again - MY TASTES - no flaming here).
Canon has less shadow noise - even at low ISO. If you underexpose a Nikon studio shot by 3/4 stop or more at ISO 200 the shadows are a bit messy with color noise. I know you shouldn't underexpose in the studio but this also applies to existing light and we all make mistakes. If you miss exposure with Nikon, even at ISO 200, there is shadow noise which can be troublesome when printing.
Both shots print beautifully at 13x19. BOTH cameras are wonderful and I'd be happy with either one. I went with Canon for very specific portrait reasons (and camera weight which is a big difference). The Nikon 13x19 print is just as sharp as the Canon print with "half" the pixels.
So let's put the following mythological creatures to rest at the bottom of the ocean:
Canon is over processed in-camera and soft - bunk.
Nikon has too few pixels to compete with Canon - bunk.
Brand Identification:
Shot 1 - Nikon
Shot 2 - Canon
Hope this is useful.