The Hardrock is below the Rockhopper in the line up, at least some years it is straight gauge Cro-mo tubing when the Rockhopper is double butted. There will be a difference in components. I don't know on Specialized specifically (too lazy to check catalogs) but what I have found with older mountain bike is lower end bikes no longer have stainless steel spokes so they rust, as you get farther up the line, not an issue. Edit: Re-read OP, clearly said "frame"