If you're applying for engineering, take the IIC.
As for the IC being easier, depends how you look at it. Because it's easier, people get better scores on it and so the "curve" is much higher. A 790 might only be the 95% for instance. Since the IIC is more difficult, people get worse scores and so the "curve" is much lower. A 790 on the IIC might be the 99%. I actually did much better on the IIC than the IC when I was in high school. I suggest you take them both and keep in mind the percentile as well as the score.