As pointed out, your 18-70 lens is a consumer zoom, though very good for the price. I guess what you want to ask is whether or not you are willing to pay a premium for full frame. The 5D is at least £800 more expensive than the equivalent APS sensor camera. And what do you get for the extra money? Well better high ISO performance, and not much more. Reviews will give a balanced summary.
However Canon should soon have a replacement for the 20D with at least 10MP. But I'm not sure if they have high quality zoom lenses to match?
Of course one reason to go for Canon - apart from the IMO over-rated full frame - is the image stabilization. Nikon have VR, but in less lenses.
My own choice would be Nikon because they have a high quality APS sensor body and lenses to match. I do a lot of macro work, and the MLU on Canon bodies is poorly designed. And as I don't like spending money the 5D is too much. But that's just my decision based on my needs and tastes.
Regarding the 18-200 zoom, the large zoom range (11x) will surely mean lots of aberrations. It might well be that the new Sony 10MP camera with a fixed 10x zoom will give better results (due to no mirror chamber allowing the designer more freedom). Not sure if it has IS/VR?
I've had a Nikon user who works in a camera store tell me that Nikon's ED lenses are generally as good as Canon L lenses, though I'm not sure whether to believe it. It's true the 18-70 is an inexpensive lens, but a lot of that is because it's a "reduced-frame" lens, which is much cheaper to do well. So is it as good as a typical Canon L lens??? Good question. Anyone have any comments?
And I imagine the 18-200mm lens will, as you said, mean lots of aberrations. The question is, will it be an excellent lens in the 18-70mm range (which makes it as good as what I currently have), with the 70-200mm being an extra bonus (for which I can live with some aberration, on the relatively rare occasions I use that range on a reduced-frame camera). Also, that lens has VR, which is a big plus for that 200mm end of things.
The only real advantage of full frame, at least for me, is that you get less noise when you have larger pixels, all other things being equal. Noise is indeed a significant issue for me sometimes. That's what I really don't like about fixed-lens cameras like the Sony you mention - those smaller sensors produce much more noise. As you say, though, the Canon is more $$$ - and you can buy a lot of noise reduction programs for the price difference.
Thanks for your comments, Leif.
Since you already have the Nikon 18-70 lens, I'd suggest trying to rent each of the cameras for a week and do your own tests. I just learned that my local Calumet has a rental 5D available, which I plan to take out sometime soon. If you could rent both, and the Canon 24-105 L lens, you'd be able to do comparisons that would really mean something to you. (If you do it, I'd like to hear your conclusions.)
And Eric - yeah, that's actually worth a try, once the new 18-200 Nikon lens comes out. There's a big camera store near me that will let me take a few sample shots with the various cameras in the store, and I'll probably give that a try. Now I just need to wait until that lens comes out, and Canon starts shipping the fixed 24-105 lens again. I'm waiting impatiently.