If you have no lenses from either Nikon or Canon tying you in to one or the other maker (and I'm ssuming you don't) then the two models - in terms of feature set are 'much of a muchness' in terms of image quality. The Canon has the clever 'anti-dust' sensor, although I'm not 100% sure of how useful that is (I don't have 400D, but used to have a 300D and any dust was quickly sorted by the use of an air blower).
The Nikon probably has the slightly better build quality (more metal in the build than the Canon). The biggest difference is the handling; the Canon is a physically smaller camera body, so if (like me) you have large hands, you can find your little finger 'slipping' off the grip on the right hand side (which is one reason why I bought a 30D when I upgraded from the 300D!)
The Nikon has a larger body and the control layout is less 'cluttered' than the Canon, so this may make a difference for you.
Both are very capable cameras, and in terms of picture quality you won't be disappointed with either one.
The single best thing you can do is go to your nearest camera shop and have a play with both models; handle them, take a few shots and see how each body feels to hold. You might find one of them is more comfortable for you than the other, which might form the basis of a purchasing decisiion!