For 2005 Rockhopper is substantially beefed up for Allmountain usage.

Both frames are using A1 allu which is essentially 6061T5 , now it seems to me that the Rockhopper has a bit more attention to detail into it and maybe some better engineering into the frame.

For pre 2005 frames, then yes, the rockhopper is more xc biased and the hardrock is a more do-it-all frame.