I have run both CIM and Chicago.
As noted by others, CIM is similar to Boston, only that it lacks the Newton hills and is much more forgiving in the last 6 miles.
On a perfect, overcast, windless 40 degree morning, I think Chicago is the faster course of the two. CIM just has a better chance of good weather, due to the time of year and the location.
It's worth noting that elevation loss is not the best measure of how fast a marathon course is. Big Sur marathon and CIM have almost identical net drops, yet one is clearly faster than the other.