Here it is, another layman's technology review. Read with caution because I've never actually even seen or used a Canon XTi.
I've been biased towards Nikon cameras for some time, but there probably isn't any real reason to pick one over the other if you are starting out in dSLR. Certainly if you already had some Nikon compatible lenses your decision would swing towards Nikon. That would probably be the easiest way to decide between these cameras.
I'm not even sure if the D50 is a current model for Nikon, it may have been superseded by the D40. The D40 is quite similar I understand. I think its metering isn't as good as the D50 and it is a bit cheaper. I think they both come with the same lens.
I've had a few Nikons and Canons for comparison of the general feel. I have owned a Nikon N2020 (one of the first autofocus slrs), an FE2 (manual) and a F601. I still have an FM2. The 601 was stolen and I replaced it with a Canon ElanII. The F601 and ElanII were the mid 90's equivalents of the D50 and XTi. This is where my main bias comes into play. The ElanII had great reviews and good feature set. I didn't like it very much. The body had too much plastic, it wasn't all that comfortable for me, and I had problems with it's exposure meter. Over time I would have learned the ins and outs with the Canon metering and that would become less of an issue. Luckily for me the camera was stolen and I replaced it with a Nikon N80, which is basically the last series of film slrs made by Nikon. I just sold this camera, but it was awesome. This opinion has been extended to my D50 which is very similar, although a bit cheaper build. It wouldn't be a bad idea to hold both cameras and see which feels better in your hand. If you have particularly big or small hands then this could make a big difference for preference.
I've never tried out the canon dSLR. I don't think the previous rebels were all that good, but it sounds like the xti has greatly improved upon those.
As for features, I think both cameras would be about the same. Typically the Canon dSLRs have better resolution that the Nikons. There are a couple of features that I always tell people to check out when comparing digital cameras. Shutter lag is one of them, but this doesn't apply to dSLRs. I would definitely compare focus speed and low-light focusing ability. These two things will always be important for your photography, and they'll make a much bigger difference on your day-to-day results rather than things like megapixels. I've also read that the Canon dSLRs can take longer to set up just right for proper exposure. This sort of matches my experience with my old Elan II. Sometimes I read about people preferring one camera over the other because of the type of memory card it uses. I wouldn't even think twice about memory card type and costs, all these cards cost about $35 to $45 for 1gb here in Vancouver.
Another thing to keep in mind is expandability. I know that both a friend and myself are quite keen to get the Nikon 18-200 zoom. By all accounts this lens is incredible, and it is the ultimate in zoom range imo. In film terms, I've always tried to cover the 24mm to 200mm range. 300mm range is better. In digital terms, this is 18 to 200. I have this covered with two zooms, but to have it all together in one lens would be nice. I've never even changed to my 75-300 zoom yet. The Nikon 18-200 lens is so hot that it's been on bacorder for a few months and no-one knows when they'll be available again. Wanting this lens is another way to decide between the cameras. I don't know much about external flashes, although I bought a used vivitar on ebay. They can be useful, if you care enough about that stuff to lug a flash around. The battery life on my Nikon is outstanding, I've only charged the battery twice since I got it.
6. all else is probably so similar that it doesn't matter.
And if I come across as anti-canon, I really am not. I have their first ultra compact powershot, an SD100. I still love it, and I hope to get the SD600 sometime.