The Signature Edition looks much better than the Special Edition. The Signature Edition has more saturated color and much better black level and contrast. The Special Edition is much brighter overall. None of the many night scenes look like night in the Special Edition. They do in the Signature Edition.
The opening where Hogarth goes on the roof to check the antenna and then into the woods with the flashlight has much more suspense, as it appears to be night in the Signature Edition. The glowing power station and the Iron Giant getting shocked by the wires has much more pop in the Signature Edition. Hogarth coming up the stairs with the TV lighting the stairwell when he goes to check on the noise upstairs while watching TV is another good example. The Signature Edtiion lighting looks like a stairwell being lit only by the flickering TV screen.
The Signature Edition is dramaticallly better. It is like a different movie. And the sound is moderately better on SIgnature Edition.