I currently use a D70 (though probably will upgrade to a D200 when it comes out ), mostly with the 18-70mm lens that comes with the kit version. It's an excellent camera for the money, and that lens is truly excellent for a non-professional-priced lens, and covers a very useful zoom range for landscape photography (the equivalent of 27-105mm for a full-frame or film camera). I found it easy to learn to use well. You can't go wrong with one.
Before the D70, I had a Canon film camera, so I'm not particularly biased toward either manufacturer. I haven't used a 20D, though, so I can't compare the cameras for you. Why I chose the Nikon is that Canon didn't have a lightweight zoom lens that covers roughly that focal range that was good enough quality for me; I used to use the 28-135mm IS lens with the Canon, but it was badly soft in the corners compared with the Nikon lens. Instead of looking at the D70 vs. the 20D, I'd recommend looking at what lens (or lenses) you like the best and let that drive the camera decision, as the lens probably has a bigger impact on image quality than the camera itself.
Of course, Canon just came out with a 24-105mm lens that wasn't available when I was looking, but it sounds like it has some serious weaknesses in addition to its strengths (and is quite expensive).
Lisa