What Rick said:
The bottom line is that Nikon D2x or Canon1DsMkII will produce superior
quality images. You can't go wrong with either.
Having used Nikons including the D2x hard under very rigorous hot/cold/wet/sandy/humid (not all at once) conditions I am kind of surprised to hear the suggestion that the Canon might be stronger. Suffice to say that in my experience the top Nikons are extremely durable and weather/abuse resistant and have a 50+ year reputation as such. I consider them bombproof.
The 1DsMkII is superior at high ISO (800+) shooting, and Canon has a T/S lens selection Nikon lacks.
The EOS and the D2x are both very excellent cameras. Some things I find better with the D2x:
Image quality.
As far as I know prints from Canon's best and Nikon's best are similarly excellent. In spite of the EOS's ~50% higher cost and additional pixels it does not, AFAIK, provide significantly better prints. Nikon worked some pretty good engineering to achieve that.
Speed.
The D2x shoots RAW at 5 to 8 fps versus the EOS Mark II 1Ds 4 fps. When you are trying to capture the "perfect" instant of a rapidly changing scene higher frame rates increase the likelihood of catching the perfect moment. Or if you are auto-bracketing, faster speed captures your bracketed images before the scene changes. The concept of shooting a burst of 5-20 frames just to catch the very best pic is new to most photogs, because with film it was impractical. With cameras like these DSLRs and fast large capacity CF cards new photo workflows have evolved.
8 fps Crop Mode.
The D2x will shoot 6 megapixel images taken from the center of the frame at 8 fps. So if one wants to shoot tele action shots like sports it is ideal.
Buffer capacity.
IMO a much bigger issue than fast frame rate is the buffer capacity at that fast frame rate. The D2x captures about twice as many pix (~22) as the EOS does before the buffer fills.
Card capacity.
Canon built a slower camera with 30% more pixels and that leads to less images per card, slower file transfers, etc.
LCD size.
The D2x LCD is more than 50% larger than the EOS. The larger LCD changes everything as far as the ergonomics of DSLR workflow are concerned. Just try a D2x side-by-side against the much smaller EOS LCD. This is a significant issue. Shooting with the lame 2 inch EOS LCD defeats some of the value of DSLR.
Weight.
Being a 200# male with large hands I don't worry about size and weight too much. The Canon does weigh almost a pound more. Dimensions are pretty similar, with the Nikon a few mm larger.
Self-timer.
Nikon has choice of 2/5/10/20 seconds whereas Canon is only 2 or 20 seconds. I find 2 seconds is sometimes not enough to let the camera settle down on the tripod if it is extended, so typically I use 5 seconds. Using 20 seconds as a cable shutter release substitute would drive me crazy. Often this is moot because I almost always have a cable shutter release with me.
Framing.
Some folks prefer a DSLR to be the same lens factor as 35mm film. Other folks do not. Quality superwide lenses are achieved at a $500-$1500 cost while supertelephoto is a $5000-$15000 cost; personally I prefer the 1.5 factor since it extends the expensive telephoto range. However, 1:1 versus 1:1.5 is a matter of personal preference rather than one being "better" unless print quality were to suffer.
The framing issue can be a big deal at the tele end. When your fast 400mm lens can function like a 600mm that is pretty significant. At the wide end I find the Nikon 10.5mm true fisheye NC correctable to rectilinear to be pretty nice, and the Nikon 12-24 deals very well functioning as 18-36 mm. So the wide end is well covered with just two quality Nikon lenses.
Ergonomics.
Try both cameras and see what you like. IMO individual ergonomics are of extreme importance, perhaps more than all other characteristics.
Regarding:
there has to be a reason why Canon is so popular with long lens users...
That occurred well prior to the DSLR era, largely because Nikon got behind in the area of long lens performance for a while. A lens discrepancy for DSLR photography does not exist today. Nikon has excellent 200, 300, 400, 500, 600, 70-200 and 200-400 offerings that work well with the D2x. And 3 quite good telextenders as well. [Note that after applying the 1.5 framing factor the lenses above function like 300, 450, 750, 900, 105-300 and 300-450 at no loss of lens speed.]
Both brands have great lenses available across the normal to mild tele range.