Are those aluminum, or brass?
Here's the style that my boxster came with. Suspect they are tecquipment, and pretty sure they are chrome-plated plastic. They are lighter weight than any real (solid) metal valve stem cap.
My main complaint is that they way they install, they don't thread on very far, so they end up kind of sticking out.
I ended up buying a few aftermarket metal ones to try, not sure I liked any better than plain cheap, black plastic. The brass ones (on the right) fit well, but were *really* heavy. (Seem to have no place on a German sports car IMO.) The aluminum ones (on the left) were heavier than plastic, lighter than brass, needed trimming to fit nicely, and would probably corrode (although I like jfp's suggestion of anti-seize... not sure why I never thought of using it on valve stem caps before!)
