St. Mark was not an apostle, by the way. He was a follower of St. Peter.
This, I think, highlights the error of the way you're argument above follows. St. Mark provides us with Christ's words as well, though he was neither apostle nor "eyewitness" (used in the strict, natural sense that he was not living and travelling with Jesus and the apostles).
What you are trying to say (if I am reading it correctly) is that a primacy should be given to Matthew's account not simply because an apostle wrote it, but because they are the words of Christ Himself.
What I am saying is that there are 4 books of the Gospels - Matthew, Mark, Luke and John.
Matthew was an eyewitness. He quotes our Lord. How is it that the words he quotes have so little meaning to the protestants? Rather than believe and accept what Christ, Himself said, as is recorded by the Matthew, seem to be of less importance than a few words written by St. Paul which are not directly quoted from Christ. Does that make any sense?