Lisa, I don't know of any such Nikon/Canon lens comparisons that are really useful, as opposed to those that are just based on off-the-cuff opinions. However, given the price difference, it would be surprising if the Canon set-up isn't at least a little bit better in some respect, at least with the lenses that you mention. The D200 ($1700) plus Nikon 18-70 ($340) is about $2040. A Canon 5d ($3700) plus Canon 24-105 ($1250) is about $4950.
The Canon is an "L" lens which is Canon's pro line and is a constant aperture lens. The Nikon, which I have, is a consumer or prosumer grade lens. For the price I consider it a good value, although optically it isn't quite as nice as Nikon's 28-105mm AF (also a similarly priced consumer lens). AFAIK, Nikon doesn't make a pro grade (fast, constant aperture) zoom lens that covers the entire range you're interested in. Nikon's pro zoom lenses tend to cover slightly shorter ranges of focal lengths but offer wider constant apertures (f2.
than the Canon 24-105. Probably the closest pro grade Nikon lens is the AFS 17-55mm f/2.8 ED, which is a DX digital lens. It has top notch optics and a fast constant aperture but not the full zoom range you want (roughly 25-83mm as a full frame equivalent).
I've used the 18-70mm Nikon on my D2X and it does fine (i.e., it doesn't have a problem with the 12 MP resolution and has excellent color and contrast) but as you probably know from using it on your D70 it suffers from very slight CA (easily correctable in the RAW conversion or in PS) and very slight distortion at the extreme ends of the zoom range (also correctable).
The interesting comparison will be to see how the 5D and D200 stack up against each other when both have equivalent (pro) quality lenses mounted. I assume that when dpreview does its review of the D200 there will be such a comparison included, but it may not feature the lenses that you're interested in.
Good luck finding useful Canon/Nikon comparisons, but even if you do, I think you'll have to do some serious thinking about the price difference unless you are a lot more indifferent to price than me.