These best way to determine one woods "hardness" relative to anothers is to look at the known specific gravity of the wood species. A terrific book for woodworkers is World Woods In Colour by William A. Lincoln. Great colur plates and tons of information including specific gravity - not cheap but worth every penny.
David, hard maple is usually a term used for Acer saccharum, sugar maple or rock maple. Soft maple is used to refer to all the other maples including your spectacular bigleaf maple.