Personally i wouldn't switch systems based on a comparison of the D70 and 20D, because whatever miniscule differences there are between the two aren't worth the hassle of switching lenses, flashes, sync cords, qr plates etc. and besides, things are bound to change anyway on a yearly basis with new models coming out.
For what it's worth, I shoot a 10D now and have no desire to trade up to a 20D, the real-world increase in resolution is minimal and Canon still has some issues with the autofocus performance in their non 1-series cameras.
I started out shooting Nikon but switched to Canon for no other reason than at the time Nikon had nothing to compare to the D30 and the long telephoto IS primes for wildlife and I needed to go digital for newspaper photojournalism. I have been exceedingly pleased with Canon, but I think the decision to switch kind of depends on what you want to do with the equipment.
These are just my opinions, but a lot of phtographers will tell you the same thing, although I love the L series zooms and telephotos (I have an exceedingly sharp copy of the 70-200 2.8 IS that is prime sharp wide open all the way through the range) I have always preferred Nikon's wide-angle lenses. Don't get me wrong, I have a canon 17-40 and love it, but I've also gone through several wide-angle Canon primes that just didn't quite measure up (In my mind) to the Nikon stuff i used to shoot.
I guess it depends on what you want to do with it. I still shoot a lot of low-light stuff and wildife so the IS is still a major deciding factor for me, but in the past couple years I've really gotten into storm photography and I miss those Nikon primes.
