The Old Testament predicts a saviour figure who it calls the Son of Man. Jesus talked about this figure as if he were expecting him to come. Christianity says that Jesus himself was this Son of Man. If so, then why was it his favourite title and not Son of God?

In one verse in the Matthew Gospel, 10:23, Jesus tells his disciples that they must flee to another town if persecuted in one town and that they will not have travelled through the towns of Israel before the Son of Man comes. F.F.Bruce says that this means that Israel will never be fully evangelised or preached to until the Son of Man comes back in the second coming (109, The Hard Sayings of Jesus). But the line before it says they will run from town to town until the coming to hide so Bruce is taking it out of context. This prophecy was not fulfilled because though the disciples did do a lot of hiding and running Jesus did not come back in their lifetimes.
It is useless to contend that what Jesus means by comes is his own come-back from the dead for he spoke of a great persecution that did not happen at that particular time or before it. Jesus has to mean the triumphant second coming on the clouds of Heaven at the end of the world. Anyway, he says so much to the apostles and others about the Son of Man coming - and considering that Christians say he meant himself - that an interesting possibility is worth mentioning. The apostles did expect Jesus to appear in glory but he died. If the Christian claim that the apostles didn't expect Jesus to rise from the dead is true then the apostles were lying. They didn't want people saying they wanted Jesus back so much as Jesus promised that they imagined the visions of the risen Jesus. They didn't want people to say, "They were urged by Jesus to expect him to appear in glory so that when he died this forced them to want him back so much that they actually thought they saw and spoke with him after his death as a resurrected being."

Bruce assumes that Mark 13:10 which says that the gospel will be proclaimed to all people before Jesus comes back at the end of the world is a parallel or it shows the meaning of Matthew. But there is no evidence of that. It is not a perfect parallel so it is not necessarily further light and anyway Mark wrote first and the person who writes first is the one to take seriously unless he is an obvious fake. Matthew meant Israel not all people.
NT Wright says the Matthew verse does not mean the second coming of Jesus but is referring to the persecutions that the Romans will start. Yes it is referring to these persecutions but he never says then what Jesus meant by before the Son of Man comes! He simply denies that it means the last judgement and gives no reasons to support his interpretation (page 10, Crisis at Clonmacnoise).
Wright says as well that the early Church used apocalyptic end of the world imagery and the image of the imminence of the end but didn't mean it literally. They used all this imagery to express the thought that the way the world was then was coming to an end, not that the world would be destroyed and Jesus would come to judge. That is just a cop-out to get around the fact that the New Testament expected the end of the universe soon and was wrong. There is no New Testament verse that supports the non-literal interpretation. Wright is forced to use Clement and Ignatius's not speaking as if the world was about to end to justify his interpretation. He mentions Justin and Tertullian not being concerned with the timing of the end. But Clement and Ignatius wrote when the Christians stopped stressing that Jesus was due in days or weeks. The Church probably felt that the end would take place before the apostles all died. They were dead then. Justin and Tertullian not caring about the timing of the end does not mean they thought it could not happen any minute. The early Church did not care about timing either - it was enough for it to believe it would happen at some unexpected time soon.
Wright also argued that first century Jews did not expect the world to vanish but the world order to break down so that there would be big changes. He thinks the Christian scriptures were using apocalyptic imagery to convey the same expectation. This is nonsense for Jesus poured scorn on the traditions that the Jews had added to the faith.
If Wright is right that Jesus only meant that God would cause great changes and world upheaval and the things he said about this being accompanied by the sun not shedding its light and the moon turning to blood seems to speak of near universal apostasy. They are symbols of spiritual darkness. Jesus spoke of the end he categorised towns in Jerusalem such as Capernaum with the pagan towns of Sodom that God delivered a judgemental destruction to. He called Jerusalem a harlot thus putting it in the same category. He didn't deny that Judaism was the one true faith but because of its spiritual slackness and because of its adding in traditions he didn't like he gave it a very severe condemnation. Jesus then was no ecumenist! If he was that dogmatic and sectarian then Protestants and Catholics should not even try to have any religious relationship.
So Jesus predicted the coming of the Son of Man. Is Jesus saying that he is not the Son of Man and that somebody else is? But nobody in the apostolic times claimed to be this personage. So if Jesus was not Son of Man and falsely predicted the coming of the Son of Man then he was a liar and a fake prophet.
Most of the time Jesus spoke of the Son of Man, he talked as if he were another person. In a few verses he said that the Son of Man would be handed over to the enemies and be crucified and rise again. But these verses are clearly prophecies written after the event or alleged event and so we can't depend on them. Jesus was asked at his trial if he was the Messiah the Son of God and he replied yes and that the son of man would be seen coming on the clouds of heaven in great glory. The Jews took this to be unbearable blasphemy. But there was nothing at all blasphemous in what Jesus said. The story seems dubious as well. And even then it is possible to hold that Jesus was being ambiguous. He could be interpreted as saying he was the messiah but the son of man would come on the clouds in great glory meaning somebody else. Or saying he was the son of man. Ben adam or son of man just means a man. The way Jesus uses the expression would be too strange unless Jesus meant the son of man was somebody other than himself.
As the son of man didn't come the Christians would have needed to pretend Jesus was the son of man.
The Jews, according to the gospels, said that Jesus cried out for Elijah to save him when he was on the cross. Elijah was possibly the son of man. John the Baptist was declared to be Elijah in the New Testament. John was dead. Jesus possibly wanted John to rise from the dead and come to rescue him. John who came without doing miracles and who claimed to be a humble prophet would be a far better candidate for the humble title of son of man than Jesus.