Circe is a danger to Odysseus and his men because she is a sorceress who drugs men and changes them in to animals. In the Odyssey, some of Odysseus's crew find Circe's mansion hidden in the woods and are given a delicious meal that has been drugged; after, she turns them all into pigs. One man, who did not eat the food, returns to Odysseus and tells him about his crew. Odysseus is able to free his men, taking the advice of the god Hermes, who told him of an herb that would protect him from Circe's magic.
Circe is more dangerous that the cyclops, in my opinion, because she is able to seduce men and hold them prisoner. Odysseus was also held prisoner by Polyphemus, but was able to escape. Circe, on the other hand, is able to detain Odysseus, who feasted with her for an entire year. Perhaps this is a case where pleasure is more dangerous than the possibility of being eaten by a giant!
In answer to your first question, Circe is mainly a danger to Odysseus and his crew because she and her underlings are a distraction. Odysseus cannot waste any more time getting home. The reader discovers what has been happening to Penelope and Telemachus while Odysseus has been absent, making Odysseus's soon return even more significant. Circe is primarily a danger to Odysseus not just as a distraction but also as someone who takes away his focus on and devotion to his wife.
Most would argue that Circe is more of a danger than Polyphemus the Cyclops. First, the Cyclops is easily recognized as a danger by Odysseus and his men. Circe is a subtle danger that many characters do not even see until it is too late. Moreover, Odysseus and his men are trained for handling physical dangers such as Polyphemus, and Odysseus easily outsmarts the giant. Circe, however, represents a psychological and emotional enemy who is armed with wit and beauty. Odysseus especially is rusty when confronting such a danger.