I have a lamb CM and have a calf 4-zip on the way. Have worn the lamb several times and it does not feel at all flimsy the way that H&M/Uniqlo jackets do. I think Drew has said that the thickness of the 2 leathers is the same, with twice the density on calf. I ordered calf not for durability/toughness, but so that I have 2 jackets with different types of patina.
These are the pics that convinced me that having both wouldn't be redundant: