The Rockhopper has slightly older geometry, and is more suited for general riding, has harder/tougher compund tires. The Fathom has the more modern longer front center geometry which has improved stability and capability of xc/trail bikes in the last few years. The spec for the giant is way better, and they have internal cable routing for a seatpost dropper which you might want to add later. The Fathom is a more serious xc bike than the Rockhopper.