I recently went the Nikon D80 route because I liked its menus and features, as well as the way it felt in my hand better than the Canons. I would, however, have gotten the Canon XTi rather than a Nikon D40 or D40x, because the lens selection is so much greater, and the Canon 105 mm macro lens is a show-stopper at a reasonable price. (Please note..."reasonable" is in the eyes of the beholder. I think the Canon 105 runs in the $450-550 on most sites; the Nikon 105 mm macro seems to be $800-1100! (Ouch!)
Sigma and Tamron have (reputedly) excellent 105 and 90 mm macros, respectively that are in the $400 range.
I would echo SuperDave's remarks in other threads and suggest one of the longer macro lenses over the 55 and 60 mm examples on the market to ensure you have adequate working distance for lighting when doing coin photography.
Remember that a digital SLR has a sensor that is smaller than the film size of a 35mm camera. Thus a 60 mm lens on a dSLR is equivalent to a 90 mm lens on a film camera; a 100 mm lens is equivalent to a 150 mm lens, and so on.
Back to the cameras...Canon's "firmware" (internal software) tends to be more "aggressive" than Nikon's and produces slightly sharper images with more vivid colors. Both manufacturer's midline and high end cameras allow the photographer a great deal of latitude in adjusting the default settings, so you can achieve the look you want. For coin photography, that probably means you'll need to do more manipulating of the Nikon's set-up than the Canon's. On the other hand, for portraits of people, you might like the Nikon's "out-of-the-box" image quality better. (It's all very, very relative and subjective...both makers' dSLR's can product fantastic photos in the right hands!
On the photography forums it seems that many people are unhappy with the "kit lens" that is common on the Canon XT and XTi, and happier with the quality of what normally comes with the Nikon. If possible, read up on the available lenses. You may, in the end, decide to get just the body, and forego the kit lens entirely in favor of one more suited to your style. There's a forum call DPReview that is chock full of useful information.
Best part of dSLR...you can take many, many, many, many photos as you learn how to use your camera with almost no cost beyond your time! And then when you get a new lens...you can do it all over again.
Just some thoughts.