He said to him, "Most assuredly, I tell you, hereafter you will see heaven opened, and the angels of God ascending and descending on the Son of Man."
The emphasis upon "Son of man" here, rather than upon "Son of God" was probably due to Jesus' purpose of reserving emphasis on the latter until the time of Peter's confession  (Matthew 16:13f).  The meaning of both titles carries the implication of Christ's deity; but "Son of God," in the popular mind, was too closely associated with "King of Israel," in the exact manner of Nathaniel's confession; and it was not time for Jesus to challenge the Pharisees by using "Son of God."  A little further attention to the title Son of man is in order.