Just to put this into perspective, this is a 4.8% difference in sensor area. Or, linearly, it's 2.3% difference in crop factor.

This is not very much, and generally _other_ measurement tolerances will be less precise. For example, if you measure the [actual focal length](http://photo.stackexchange.com/questions/13603/how-to-test-actual-focal-length) of, say, a bunch of different models of 50mm lenses, they probably have a _greater_ variation in field of view.

In general, newer sensor technology moves forward, and in this case there's no exception: the D3100's sensor is significantly better, particularly for controlling noise at high ISOs. From [dpreview][1]:

> The D3100 offers little to complain about in terms of image quality, and its new 14Mp sensor delivers very good results. High ISO performance is substantially improved over the D3000 [...].

Basically, the small difference in sensor size is insignificant compared to the improvements in terms of image quality, and the small difference in framing is likewise a non-issue.

  [1]: http://www.dpreview.com/reviews/nikond3100/page19.asp