They do the same thing. Forerunner slipspace can tear a hole in reality but not something like the UNSC engine which shortens the physical distance. It takes months to cross across the galaxy with a slipspace engine yet it takes hyperspace only a few days. The Rakatans created hyperspace feuled on a planet's force potential. A missile able to go as fast as to bend reality with hyperspace would go strait through a ship in slipspace.