“What shall I do, then, with the one you call the king of the Jews?” Pilate asked them.
(Mark 15:12)
...and Matthew 27:17:
So when the crowd had gathered, Pilate asked them, “Which one do you want me to release to you: Jesus Barabbas, or Jesus who is called the Christ?
Under the assumption that here 'King of the Jews" = "Christ'' , the difference would be absolutely nothing: there would be no need of starting this thread.
But in another thread, I have pointed which is probably the marcionite meaning of the Barabbas episode:
if A -----> B then not B -----> not A
So if Jesus 'called king of the Jews' is really the 'king of the Jews', then 'Jesus Barabbas' is really the marcionite 'Jesus the Son of Father'.
But Jesus Barabbas is not really the marcionite 'Jesus the Son of Father'.
Therefore Jesus 'called king of the Jews' is not really the 'king of the Jews'.
So if Jesus 'called king of the Jews' is really the 'king of the Jews', then 'Jesus Barabbas' is really the marcionite 'Jesus the Son of Father'.
But Jesus Barabbas is not really the marcionite 'Jesus the Son of Father'.
Therefore Jesus 'called king of the Jews' is not really the 'king of the Jews'.
Now, if Mark is separationist, Jesus couldn't be called 'Christ' in a Gospel where he is not really the (Jewish) Christ. Even if Jesus is really possessed by "Christ". There is the difference here between the Christ who is possessing the man Jesus (this Christ is not the Jewish Christ) and the title 'Christ' for the Jewish 'king of the Jews'.
My suspicion is that Matthew changed the markan 'called king of the Jews' in 15:12 into the his 'called Christ' in 27:17, in order to make clear and explicit that the man Jesus is stricto sensu the (Jewish) Christ, without no clue of separationism at work. Even more so: without the risk that 'Christ' served to name an entity different from the 'king of the Jews'.
So in Mark, "Jesus" is really possessed by "Christ", but neither "Jesus" nor "Christ" is really the 'king of the Jews', the Jewish Messiah.
In this way, Matthew could neutralize in advance the marcionite point behind the Barabbas episode and continue to preserve it in the his gospel: the important thing for him is the identity Jesus = Christ. Once assured that identity, the Jewishness of the 'Christ' in question is merely a collateral effect.