The Rebel is by far the most popular Digital SLR, according to a variety of sites. Digital Preview has it at double the clicks of the other two, and CNET has the four variants of the standard package above any other camera. It gets the best reviews of the trio, with the 20D being in the far shadow.
The main reason for the gushing seems to be that the camera is the cheapest of the three, by some $200-$300, depending on version and variant. It is also much lighter (by 100g) than the other two, and it boasts the most accessories, thanks to wider audience.
Its relationship with its bigger sibling, the 20D, is studied on the corresponding page. Suffice to say that the two are marginally different according to the specs, but the 20D counts as professional SLR, while the Digital Rebel is a prosumer. If the label is the whole difference…
The Nikon D70s, on the other hand, was placed squarely against the Digital Rebel. That is, except for the much higher price, that reminds us much more of the bigger sister. As a matter of fact, the D70s and the D20 are very close.
So far, what speaks in favor of the Digital Rebel is its popularity. Although I am reminded of the German saying: "Eat shit! Billions of flies can't all be wrong!"