I don't know about the valve covers. You want the one which uses the thickest metal. I would not be surprised if they are the exact same thing.
The stud is 5/16 coarse thread for the head, fine thread for the nut. There is not a stainless stud I am aware of. SS and chrome acorn nuts can usually be found at a good local hardware store. The torque spec for the valve cover nuts is 2 - 2 1/2 ftlbs (24 - 30 inlbs). Don't over tighten them.