Below you can see the front view size comparison of Nikon D80 and Nikon D90.
Weight is another important factor especially when deciding on a camera that you want to carry with you all day.
Nikon D80 is 35g lighter than the Nikon D90 but we don't think this will make a significant difference.
Also keep in mind that body weight is not the only deciding factor when comparing two interchangeable camera bodies, you have to also take into account the lenses that you will be using with these bodies. Since both Nikon D80 and Nikon D90 have the same APS-C sized sensor, their lenses for a similar focal length and aperture will be similar in size and weight.