Here is how the two cameras compare in lab tests
http://www.dxomark.com/index.php/Camera-Sensor/Compare/Compare-sensors/(appareil1)/645%7C0/(appareil2)/664%7C0/(onglet)/0/(brand)/Canon/(brand2)/Nikonthe D3100 has a slight edge, but also has better low light performance. Either way, both a great cameras and will keep you going for years to come.
I would suggest the D3100 because it is $200 cheaper, and you can put that $200 towards accessories or another lens.
Now if you wanted to, you could wait and save a little more for the Nikon D5100 which has a much more powerful sensor than both the D3100 and 550D, the same sensor found in the Nikon D7000 ($1500)