Both these cameras are fantastic, but I think you'll like the Sony A35 a lot, it has great video capabilities (including phase detection autofocus, which the D3100 doesn't do), and great image quality and as you pointed out rapid fire.
The D3100 has a slight edge in image quality at high ISO, but they are very close. The A35 has an awesome twilight mode which takes several photos in rapid succession at a high ISO and combines them together to remove the noise resulting in a good quality shot in very low light. Nikon doesn't tend to have innovative features like this.


