The History of Nikon and Canon has been an interesting one, but let's talk about the HERE & NOW. Up until the release of the Nikon D300 and D3, Canon had a sizable lead as their sensors could shoot in less light with less noise. Nikon hung their hat on better ergonomics and focusing but that wasn't enough to sway any Canon fans.
Enter the D300 and D3. With one fell engineering swoop Nikon took low light shooting further than any other DSLR maker and then added a full set of professional features to both cameras. High ISO shots from the D300 were impressive for a DX camera, but samples from the D3 appeared too good to be true. But they were true and Canon has been fighting to regain some momentum ever since.
Not long after the D3 was released Nikon announced the D700, the standard sized version of the D3. Unlike Canon, Nikon did not cripple the smaller D700 and in spite of a much lower cost it was fully featured and capable of 8 FPS with a fully pro focus module. Compare that to Canon's recent release of the 5D MKII, an amazing camera that they chose to cripple with slower shooting and consumer level focusing. It's still a terrific camera, but Canon does not have the "give 'em the kitchen sink" attitude that Nikon does.
But cameras aren't everything and both companies have impressive lenses. For the moment Canon has more good primes an Nikon has the better zooms. Few would argue that the Nikon flash and metering system is better or that the auto focus modules more adaptable.
The Bottom Line
It may sound like I think Nikon is WAY ahead, but in fact I don't. The REAL WORLD differences between these cameras are TINY. And those differences will effect shooters in different ways, so there's never a clear winner. For now the image quality of ALL DSLRs is terrific. Period. So it's safe to choose a camera based on features, lens availability and even weight. Find what works best in your hands and fits your budget. A talented photographer will get great results with a D40, while some gear hound with a more expensive camera will not do as well. Working on your craft is far more important than the actual camera you choose!
