Having just bought the D70, I may be biassed. As cameras, I would agree the D70 & 350D are pretty equal, but I would choose the 350D over the D50.
I don't know the advantages of the EOS 350D (apart from lighter in weight), so you'll have to look elsewhere for that, but I would say that at this precise instant in time, the D70 has the following advantages:
£100 cheaper with cashback (see my other thread)
better kit lens (18-70 that is)
better flash system & sync
(And I have a personal long term preference for Nikon as a company, despite the fact that Canon is undoubtedly the technology leader these days.)
If you take the trouble to learn how to use it, the (external) flash system is significant. (But most people don't)
Nowadays, it's a fallacy to think that you need a "simpler" camera because you're new to it; the D70 (and I am sure the 350D or D50) is extremely easy to take superb pix with with no knowledge whatsoever, and absolutely far easier than any point and shoot. With the instant feedback you get with digital, and this forum to ask questions on, you can easily teach yourself the finer points.
As regards size and weight, spend £100 on something you can slip into your pocket and carry always. You'll be wanting one or two extra lenses for the SLR anyway, so the size difference isn't so significant.
Personally, I wouldn't buy a D50, but I'm sure it's OK. Differences are tabulated here (but there's a firmware upgrade for D70 that cancels some):
http://www.dpreview.com/articles/nikond50/
Kit lens, remote flash and compactflash storage are the important differences for me.