I've been thinking about our most recent Circuitous Conversations podcast where Dan and I give advice on what dSLR to buy. A question that the both of us get more often than we can count. I think that the conclusion that we came to was that you really can't go wrong with any of them anymore, even the least expensive model on each of the Nikon/Canon line. That said, I realized that we overlooked an important facet of that discussion, which is priorities. What's most important to you? While all of these cameras can capture a good picture, some do certain things better than others. For example, my priority is image quality when there's light around. To that end I use a Canon 5D Mark II. It's got a full frame sensor, 21 megapixels, and has very low noise within any reasonable range, and is reasonably small & light. While I don't agree with Dan that Nikon's UI is substantially better than Canon's (I think they both have their strengths and it comes down to what you're used to), even if I did like Nikon's UI better, I would probably still use a 5DII because image quality is my priority and 12MP full frame cameras can't compete (btw, I know about the 24MP monster but it's big, heavy and really expensive which is why I switched from the 1Ds3 to the 5D2 in the first place). I'm willing to give up something in order to get what I want.
Similarly if I were into taking pictures of birds, I'd probably go with a cropped sensor in my camera. That way you get about 50% more distance on your lenses and a bit more depth of field to boot. Or another, I never put my camera on continuous shutter. In fact, I rarely if ever take more than a picture or two a second. But if I were shooting sports, I'd need one of those hummingbird fast 10 FPS speed demons. The need for speed would trump the fact that it gave less detailed pictures.
Any camera body on the market now will do almost all these things well. They'll have plenty of pixels, enough FPS for most, be decent in low light, etc. But if you've got a specific need, and it's that important to you, that priority could easily dictate what camera you should buy. Just something to keep in mind.