Canon retains its signature strength in one area: low-light shooting. Pixel noise is virtually nonexistent up to an astonishing ISO 3200 sensitivity setting. Nikon has come a long way since its grainy days of just a few years ago. By switching from the CCD image sensor in the D80 to the CMOS chip in the D90 (the same technology that Canon's been using for years) Nikon produces far crisper photos. But images viewed at full-size still have a slight speckly look even as low as ISO 800 sensitivity.

Canon hasn't just stayed the course, though. While its photos were always smoother than Nikon's, it cameras often had a problem with color noise -- errant specks of blue, green, yellow, or (especially) magenta creeping into items that, in real life, do not contain those colors at all. Color noise essentially disappears up to ISO 3200 on the D50.
Overall, I'd say that people who photograph mainly at night would probably get better results from the Canon D50. Check these photos from election day (shot in RAW and lightly edited) for examples. But I've also gotten great nighttime results with the D90, as you can see in these unedited JPEGs of East River bridges and of circus performers at the Spiegeltent on the Manhattan waterfront.
Giving the Canon 50D a fair review was challenging because of the absolutely crappy lens it came with. The EFS 17-85mm lens Canon sent has an image stabilizer, which is nice. But it also produced rather soft (aka, slightly blurry) images and absurd amounts of color fringing--a refraction artifact, common in cheap lenses--that puts a purple, blue or green outline around any border where light and dark items meet, such as tree branches against a bright sky.

Unlike many entry-level SLRs, the 50D does not come from Canon with an official "kit" lens, although some online vendors toss in the 28-135mm lens (which I haven't tested) for around $1,400. Nikon, however does offer a kit option for the D90. You can buy the camera either by itself for around $900 or with the Nikor 18-55mm image-stabilizing lens for about $1,200. Not only does the Nikor have substantially more zoom range, it also delivers far-crisper photos, without a hint of color fringing. And $300 is a bargain for this lens. By comparison, Canon's crummy 17-85mm lens sells for between $350 and $600 online.

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It would have made far more sense to compare the 50D and the D300, since they are closer in both price and specs.
The 50D and the D90 are in two different categories. The D90 is basically a glorified budget model, with some admittedly nice features tossed in. For example, the 720p video recording, though it has some pretty major flaws, is at least on paper, a great bonus (although some would call it more of a gimmick than anything).
But while the D90 would likely be a lot of fun to use for most budding amateurs, it just doesn't offer the build quality or the image quality of the 50D. And anyone hoping to take serious video with it will be frustrated to no end. The 50D, of course, opts to pass on the video aspect and just stick to what DSLRs do best--and there's no doubt it's better at that than the D90. As for the apparent user-friendliness of the D90, much of this ease-of-use paradoxically comes from less flexibility, and options that are tucked neatly away to avoid confusing novice DSLR users.
My advice to anyone who is actually considering between these two cameras for their next purchase: before making a purchase, you may need to do some soul searching and decide precisely what your priorities and your needs are, and then determine whether you want to take the next step in DSLR photography. If you would prefer to remain more casual in the pursuit of your hobby, I can think of no better option than the D90. But if you're itching to step up, I would suggest the 50D is more up your alley. (However, note that from the reviews I've read, the Nikon D300 appears to be a slightly superior camera even to the 50D, so I would definitely recommend some hands-on time with both before making a final decision.) Hope this helps.
What a great comparison! It's hard to argue with actual printed results. While I've used and prefer Nikons since the S model for stills, I use and prefer Canons for video. I hardly believe that 720P is "more of a gimmick" - check out the author's indoor videos from his link. You should see what outdoor video is like!
Actually, the Rebel XSi (450D) comes closer in price and features (also a plastic body camera); same 12 MP (the Nikon is 4288 x 2848, the canon is 4272 x 2848), 3 inch diag LCD screen on both, competitive on lenses and accessories (including "smart" flash units); etc. - but the Nikon (with movie mode) is roughly $200 more street price right now.
The first comment ends with the suggestion that you spend some hands on time with the cameras and I would like to emphasize this as the most important - having good numbers doesn't mean a whole lot when you are uncomfortable with the camera for your usage (be it portraits, outdoors, macro / close ups, etc.
This is also where you look for tricks like wireless remote, and other accessories for your specific use. With computers I always stress your type of use dictates software dictates hardware; the same holds true for these cameras - your use dictates the camera you want.