I get emails out of the blue sometimes, and yesterday a guy who found me via Flickr asked my advice on buying his first serious digital SLR (single-lens reflex) camera. He's already quite a good photographer, with a keen eye, and he's trying to decide between the Nikon D300 and Canon EOS 40D.
He likes the Canon's interface, but prefers the features of the D300, as well as the more vivid colours it produces (at least in JPEG mode). He wants low noise, good low-light performance, and good ability to take landscapes, high dynamic range (HDR) photos, and macro (closeup) pictures.
When you're starting out buying an SLR, what you're doing is committing yourself to a camera system, particularly with lenses. Chances are you will keep and use the lenses for much longer than you use that camera. For instance, I have a Nikon D50 I bought in 2006, but my lenses include a zoom that I bought in 1995 for my old film Nikon, and which still works great. I've been using Nikon SLRs since the early 1980s—had I chosen Canon or some other camera back then, I'd probably still be using that brand, just because of the inertia.
Both Nikon and Canon are good choices—but don't forget Pentax's K20D (Pentax provides some of the best values in both SLRs and quality lenses these days) and even Sony's A700 (Sony took over Minolta's camera business, and offers very nice Zeiss lenses too, some of the best available). I'm not a fan of the four-thirds system from Olympus and Panasonic/Leica, but you might like it.
The D300 is about 50% more expensive than the 40D (around $1800 Cdn vs. $1300 Cdn), so that might make the decision for many people. (The closest competing Nikon is actually the aging D80.) One way that price difference could affect your decision might be with lenses: if you can only afford an expensive body (like the D300) with cheaper lenses, you'd be better off getting a less expensive body (like the 40D) with better lenses like Canon's top-end L series. Again, the lenses will be around your kit longer than the body.
And for any camera you get, rather than buying yourself a standard zoom lens, I would recommend getting one or two good-quality prime (non-zoom) lenses instead. They are better optically, faster (i.e. have a larger maximum aperture letting in more light), and will also make you think about your compositions more carefully.
So I said to my questioner that if he doesn't have a brand preference for other reasons, and if he can afford the Nikon D300 with a quality lens or two, then I'd recommend it over the Canon 40D. The D300 offers excellent low-light performance, an amazing LCD screen (much better than anything Canon has), and lots of other features. Not that there's anything wrong with the 40D, but by most objective measures the D300 is superior.
As far as lenses go, if I were starting from scratch, I'd get a basic fast prime like the $140 Nikon 50 mm f/1.8 or the $500 digital-only Sigma 30 mm f/1.4. For a second lens, I'd look at either a general-purpose stabilized zoom like Nikon's well-reviewed $750 18-200 mm, or (more likely in my case) a moderate telephoto prime that can also act as a macro lens, such as Nikon's $900 Micro-Nikkor 105 mm f/2.8. If I had more money to spend, I might get both of those, or go crazy and buy the $1500 manual focus Zeiss Makro-Planar 100 mm f/2.
It would be a pleasant luxury to be starting an SLR system from scratch, and to have maybe $4000 to spend on it. But no matter how you start, if you're a photography enthusiast, you'll get more lenses and other accessories (flash, battery grip, tripods, etc.) over time. Just don't cheap out with your lenses to start, and you'll be happier in the long run.