I had a D3000, and at the time had access to my dads D90, the image quality was better but not massively, the biggest advantage was the extra buttons and the way you could change settings without having to dive into menus.
However I now have a D7000 and that's is way better than the D90, especially when it comes to ISO performance.
As others have said it depends on budget but the D7000 is a massive upgrade, it did take a few weeks to get used to as its no where near as forgiving as my D3000.