The Hardrock will be fine. I think the Rockhopper frame is slightly lighter, but not much. If you want the bike to be more comfortable on the road, get the 21. If you want to be more comfortable on the trails, get the 19. I would get the disk model, but only because when you break your fork at the dirt jumps, it will be easier to get a replacement that takes a disk brake. I was recently forced to convert to a disk up front on my mountain bike because nothing was available that would accept V-brakes.