If you turn a blind eye to the cheap feel of the Nikon D3100 over the solid feel of the Nikon D90, On paper the Nikon D3100 has higher "Spec" over the elderly Nikon D90.
The motorised body is not an issue as my lenses all have the built in motor into the body.
For example the Nikon D3100 has better low light performance with higher ISO settings. The images at ISO 3200 on the Nikon D3100 are still very impressive and even ISO 6400 is still OK. On the Nikon D90 there is a lot of "digital noise" in the image at ISO 3200.and at ISO 6400 the Nikon D90 looks like the image was taken on a £50 ($80) mobile phone.
См. статью: Why is the Nikon D90 more expensive than a Nikon D3100 (Body only)?