Go with plastic.

The metal (either steel, stainless, or aluminum) will cause a chemical bond with the valve stem over time. Anytime you put dissimilar metals together you can expect a long term reaction, be it chemically "welded" or the opposite, corrosion. These negative effects are magnified by the fact that our valve stems are actually part of an electric sensor.......think electroplating, electrogalvanizing, etc.