I would buy the D3100 over the D90, newer camera better sensor, etc.
When I bought the D700, 3 years ago, it was 3000.00, and when I bought the D800 last week, it's $3000.00 also, but with 3X the pixels 12 meg vs 36 meg, plus HDV video. The D800 will be my last camera for at least 5 years now.
The D90 is 4 years old now. My D200 is 7 years old now, I only use it for underwater. Can't justify a new housing for the D800, it's tempting
http://www.bluewaterphotostore.com/sea-and-sea-mdx-d800
I have all the ports and gears for it, but it's probably going to land in at 3600.00-3800.00, then what do I do with 2 Nikon D200 bodies.
And I have 2 underwater housing for the D200.