Sensor Size on DSLR camera?

I'm looking at two different entry level DSLR's for my first DSLR camera.

I'm wondering how much difference there would be in image quality with a sensor size of 23.1 x 15.4 (Nikon D3100) versus 22.3 x 14.9 (Canon T1i)

I have a slight preference for the Canon, though am curious how much difference there would be with a slightly larger sensor in the Nikon?

Coming from a P&S I'm positive either one will be an upgrade, just curious with the sensor size, as I know with sensors bigger is usually better (or so I'm told)

While sensor size in general is important, that little bit of difference is negligible here. For comparison, full frame sensors are twice as big as APS-C ones, and APS-C ones are 15 times bigger than P&S ones. So these few percent of difference aren't going to have much impact. Note that the sensor of the D3100 is a much newer design than that of the T1i (which is a bit notorious for image noise) though, so that is a more important piece of information in this case.