It is a problematic comparison. The D70 probably wouldn't be around if it weren't for the D70.
Plus it seems that there are more Canon users around than Nikon owners, plus there is the whole, it doesn't matter what the camera, its the eyes behind it...blah blah (I get a little tired of that debate, I mean of course it isn't the camera, someone has to be there to push the friggin button!)
Anyhoo, it's problematic, and I can think of several reasons to get rid of the 300d (20d, want full sensor etc), I can't think of any reasons to get rid of D70-well maybe if it killed my father-but otherwise...
I was really thinking about Canon as well, mostly for the full size sensor in the higher end
models, not that it is an option now, but one day with high end lenses, the full size sensor may be an issue, which is an avenue that Nikon isn't taking at the moment.