To be honest, I can't say for sure since I haven't studied both of these models in detail. The difference in price is certainly attractive, and as thw, the Nikon D50 and 18-200mm VR must be a beauty.
However, I am already stuck with a Canon, so it's going to remain this way. At this time I am even saving up to upgrade one of my Canon lenses. So far it hasn't become apparent to me that going Canon was a mistake, but again, I could be ill-informed.
Beginners could certainly consider a used 300D, and I'd venture so far to say that a lot of today's DSLRs have a heap of advanced features that not every photographer will use. Hence, for a person who's just starting off, I reckon that a 300D is more than enough. Not to mention that there's a WASIA hack floating around that actually restores some of the more elusive features on the 300D, such as mirror-lockup.
When buying into a brand of cameras, also look at their lens lineup and compare the differences in prices between them. Make sure that a great range is available and at different prices and quality so that you have a chance to explore other areas of photography in the future.
Although people say that a photographer is much more important than their equipment, on the equipment side of things, the lens probably matters more than the DSLR itself.
For example, the very old D30 and D60 are still used by many photographers, and they've instead chosen to buy better lenses to allow them to gain the reach/ wideangle that they desire.
Try asking at the Nikon forums to see what they think. It's best to ask at both Canon and Nikon forums to hear both sides of the issue. Also, regardless of what some reviewers think about which brand is better value for money, you should independently read the reviews at:
www.dpreview.com
www.dcresource.com
They are very detailed, and I feel, quite impartial. Take a look and see what you really like and want before you decide to commit. It's not a cheap thing to get into.