I have the nikon d70. Its a great camera, and although it is not up to par with the canon 20D on a few features, the two of them are always compared to each other in practically every review I've read.
If you're in the nikon system, I would stick to nikon (if not, sell me your lenses, haha!) No, seriously, I think both companies make great dslr cameras, but you may like the feel of the nikon and navigating the menus a bit more with the d70. I tried out the 20d and I wasn't comfortable making the switch--Im too used to playing with nikon gear.
Both cameras will make excellent photos, no doubt about it. I am using about 10% of my nikon d70 right now. I love it, but it will take me awhile to be able to use it to its full potential.
Nikon may have something up their sleeve along the lines of a d200, which will directly compete with the Canon 20D and improve upon many of its features. People have been waiting for this "d200" for quite some time, although nikon has officially said nothing about it. But in the meanwhile, I would purchase a nikon d70, not d70s unless you need the larger LCD screen. Or you could get a cheaper d50, but I wouldnt.
The d70 with firmware upgrade acts the same as a d70s.
Good luck!
Craig