I'm sure you'll find lots of useful reviews and comparisons on the web. Be sure to read Bjorn Rorslett's review of the D2X and comparison with the 1DsII. I'll just add a few notes from my personal experience as a D2X user. Notwithstanding my bias as a Nikon owner, I'll say that truthfully you will be delighted with whatever you chose.
As you note, the big difference between the Nikon and the two Canons you mention is sensor size. For my needs, which involves quite a bit of landscape shooting, the Nikon 12-24mm lens solves the problem at the wide end. I suppose one could say "I would prefer an f/2.8 rather than f/4" but it really hasn't been an issue. Most of the time I'm more likely to be stopped down to f/8 or f/11 for depth of field anyway. Anyway, I find that whatever compromises result from lack of full frame are more than compensated for by the "multiplication factor" and the ability to have better edge to edge sharpness, avoiding the soft corners that can affect some lenses. For wildlife shots in particular, the multiplication factor can be a big plus, giving you some more affordable options for a lens with really long reach.
In the field, the D2X is a delight to use. It fits my hands comfortably and the controls are logically laid out and easy to use. The camera is incredibly responsive--the first time you press the shutter, you'll be astounded by how short the mirror black out is, even if you're accustomed to using a pro camera. The viewfinder is big and bright--I wear eyeglasses and have no problem--and I don't see any difference in the viewfinder from that in full frame pro Nikon film bodies in the past.
Image quality is great with very accurate color and good dynamic range. I know there are lots of debates about how this or that camera compares to medium format, etc. I know that for my purposes ( with a usual maximum print size of 13 x 19) I have no trouble getting prints that I regard as clearly superior to what I could do with 35 mm film.
As someone whose first serious camera was fully manual (an OM-1), I find the range of features on all of today's cameras quite wonderful. I just feel happy to be able to use such a fine camera as the D2X and don't spend much time worrying about comparisons. Good luck with your search.