The strait is narrow and so Odysseus has to pass by either Scylla or Charybdis.
Circe tells Odysseus that Charybdis is far more dangerous, as 3 times a day it sucks in all the waters around it. This would completely suck in Odysseus and his ship if he is sailing too close to Charybdis.
Circe advises Odysseus to go to Scylla's side of the strait, for despite the danger of the monster, it is preferable to lose a half-dozen men than the whole ship.
