I read every silk message that comes through. While I haven't tried it yet, you'll see the crew about evenly divided between "5M" and "8M" china silk. (They're unanimous about the 'china' part).
5M is lighter. 8M is a bit heavier, but its proponents claim it's so much easier to fill the weave that the difference is a wash by the time the plane is finished.
I'm going to get some of each, if and when I try silk.