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On its surface, the expression "Son of Man" simply refers to the humanity of Christ. The term "son of man" is used over 100 times in the Old Testament (almost all of them in Ezekiel) to refer to human beings. "Son of God" would refer to His deity. So, Jesus is identifying Himself as both God and man, which allows Him to be an acceptable sacrifice for the sins of mankind. As a man, He could die for men. As God He could be sinless, which would allow His sacrifice to be acceptable
It is interesting that in Daniel 7:13-14 the expression "Son of man" is used specifically of the Messiah. It is the one verse in the Old Testament that applies the term to the Messiah. It reads,
"I saw in the night visions, and, behold, one like the Son of man came with the clouds of heaven, and came to the Ancient of days, and they brought him near before him. And there was given him dominion, and glory, and a kingdom, that all people, nations, and languages, should serve him: his dominion is an everlasting dominion, which shall not pass away, and his kingdom that which shall not be destroyed."
So, when Jesus uses this title for Himself, He is claiming to be the Messiah. In Matthew 26:63-64 Jesus makes His claim not only of deity, but of Messiahship to the high priest, who clearly should have been familiar with the prophecy of Daniel. It says,
"But Jesus held his peace. And the high priest answered and said unto him, I adjure thee by the living God, that thou tell us whether thou be the Christ (Messiah), the Son of God (Deity). Jesus saith unto him, Thou hast said: nevertheless I say unto you, Hereafter shall ye see the Son of man (humanity, deity, Messiah) sitting on the right hand of power, and coming in the clouds of heaven."
Jesus said the same thing to His disciples in Matt 25:31:
"When the Son of man shall come in his glory, and all the holy angels with him, then shall he sit upon the throne of his glory"
Maybe Matt 16:13-17 sums it up best:
"When Jesus came into the coasts of Caesarea Philippi, he asked his disciples, saying, Whom do men say that I the Son of man am? And they said, Some say that thou art John the Baptist: some, Elias; and others, Jeremias, or one of the prophets. He saith unto them, But whom say ye that I am? And Simon Peter answered and said, Thou art the Christ, the Son of the living God. And Jesus answered and said unto him, Blessed art thou, Simon Barjona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven."
Peter equates "Son of Man" with "Christ", and calls Him the Son of God.