Compare the 50D with the D90 not the 400D, and why not the 450D or 500D models that are more current than the 400D which is 2 generations old.
The Nikon D90 is more an intermediate camera rather than a beginner model like canon's 400D or Nikons D60.
I would really compare the D40, D40x and D60 nikon models to the canon 350D, 400...450 & 500D models.
Nikons D70, D70s, D80 and D90 models against canon's 20D, 30D, 40D and 50D models
with Nikon D200, 300 and 700 models v canon's 5D MkI and II.
Peter