I ride a basic 2003 hardrock
I've ridden my friends 2002 rockhopper a number of times

What everyone has said about the rockhopper is ture, it is slightly lighter and more XC oriented, and holds almost no advantage over the hardrock for hownhill riding...but thats only becasue theyre both horrible for DH

the hardrock has a slightly stronger frame, and is cheaper, other than that it is insuperior ot the rockhopper in every way

the rockhopper is geared towards XC, the hardrock is geared towards nothing

so basicly, if you wanna do XC riding id go rockhopper, but if you wanna save the money go hardrock til you know what kinda riding exactly you wanna do, then buy a good bike