I rode the Hardrock Sport, the Rockhopper, and the Kaitai (think 29" wheel version of the piranha) I am totally sold on the Kaitai right now and just looking for a shop that has a Piranha to ride to see if I like it just as much.
Between the two for me I would say the Hopper was more rugged but less stable. the kaitai in this case had a much more comforable saddle, better shifters and derailers, and better geometry. I was on my way to go buy the Rockhopper when I decided to visit this other shop, so I really like the Rockhopper too, and for me to feel that this bike outclasses the Hopper by so much really means somthing.
But each person has a diffrent want in a bike and a diffrent fit, so really you just need to ride them back to back and see wich you like better.
Make sure you have the same size frame and the seat is adjusted the same so you get a more equal comparison.