Positives:
If you need Spot metering, or 1/500th Flash X-Sync instead of 1/200th, or +-5 Exposure Compensation instead of +-2, or somewhat better battery life, or slightly better Buffer-Full JPG shooting performance, or Front/Rear Curtain flash modes, go Nikon.
If you want slightly faster shutter release response, go D50. If you want 1/8000th sec max shutter speed, or a pentaprism instead of pentamirror, or 1/3 stops between ISO sensitivities, go D70s. Also, if you want slightly sharper images than the D50 (in exchange for more moire), go D70s.
If you need more ISO versatility, or higher sensitivity for better low-light shooting (Canon Rebel XT's ISO100 is almost as sensitive as Nikon D50's ISO200), or better absolute image quality (though very slightly softer at ISO1600), or 8 megapixel resolution instead of 6, or a bigger buffer allowing more shots to be taken in burst mode, or RAW burst capability, or light weight, or faster card write speeds, go Canon Digital Rebel XT.
Negatives:
If you want images with extreme moire (chromatic rainbow-like blending effect on very small patterns, such as fabric textures or other tiny details near the extinction point), go D70s, or, to a lesser degree (because it's softer en general), the D50. Canon's Rebel XT has NO moire whatsoever.
If you want a cheaper, less well-constructed build, go Digital Rebel XT. Nikon's low-end dSLRs are sturdier and better built.
Other Info
Nikon D50 is cheaper than Canon Rebel XT, while the D70s is more expensive than both. Keep in mind that lenses and memory cards will add to the price of any dSLR quite a lot. However, both Canon and Nikon make their own very inexpensive but very good 50mm f/1.8 lenses.
Supposedly (no personal experience) Nikon's 18-200mm f3.5-5.6G IF-ED AF-S DX VR Zoom-Nikkor lens is a godsend, for only about $800 or so.
There is a Sigma lens, the AF 18-200mm f/3.5-6.3 DC available for Canon AF mount, and it is much cheaper at ~$320, but the optical quality is probably not as good and it has quite strong barrel/pincushion distortion (among other issues).
There probably isn't any equivalent zoom lens available for Canon with the price point or claimed quality of the Nikon 18-200mm. Canon's 28-300mm f/3.5-5.6L IS USM lens is over $2000.