I don't think the question is so much whether the aluminum alloy, not to mention plastic or anything else, can be plated as it is which is considered "higher or lower grade".
The answer is neither, but to fit the right one for the bike. An earlier bike is best fitted with the chromium plated steel stem and a later bike with the Al alloy one. I don't know if the two ever ran in parallel: if they did, the obvious choice on a bike of that era is the aluminum one because it is lighter and any increased flexibility is of no consequence. The aluminum ones ultimately came in several styles of course and with old and new logos, just to make it fun finding the right one for the bike in question.
Hugh Thornton Cheshire, England
In a message dated 04/01/2006 23:20:34 GMT Standard Time, email@example.com writes:
I have two Cinelli stems...one alloy (early 80s) and one that looks like chromed steel (early 70s). Are the chromed ones considered higher or lower grade stems than the alloy ones? Or are they both alloy and one has chrome applied to the alloy surface?
Los Olivos, CA