Matthew emphasizes Jesus' words, which, for some reason, he is more interested in than Mark, who emphasizes his deeds.
I think that the impression that Mark emphasises Jesus' deeds more than Matthew is largely derived from Mark's rather 'breathless' style: 'immediately Jesus did this, and then he immediately said that'. All the time giving the impression that Jesus is continually on the move. I think it is this sense of movement is really the only real difference of emphasis between the two evangelists - that's of no great consequence. The emphasis on Jesus' words in Mark is, as Jeremy says, just as important.
There are a few other details other than the ones mentioned which incline me to opt for the Markan priority view. Matthew appears to omit certain details according to his Jewish agenda. But there are other instances where you sense he's tarting up the bare details of Mark with a fair amount of technicolour enhancement. The matter of the Resurrection is a case in point. The ending of Mark is generally agreed to be a spurious addition, but the accepted genuine words relate that a young man dressed in white simply tells the women that Jesus has risen and is no longer there*. That's not good enough for Matthew: he has to depict the moment of Resurrection with an angel descending to roll away the stone, leaving the guards terrified.
And the witnesses to this were? Presumably the guards (if it happened) - who would be unlikely to divulge details of the occurrence to members of the Christian group whose leader's body they had been entrusted with guarding. In short, Matthew is quite obviously letting his imagination run riot.
*For those (probably few, in this learned forum!) who are not aware of the genuine ending of Mark's gospel, it is as follows:
(the young man in white tells them)“Do not be alarmed. You seek Jesus of Nazareth, who was crucified. He has risen; he is not here. See the place where they laid him. But go, tell his disciples and Peter that he is going before you to Galilee. There you will see him, just as he told you.” And they went out and fled from the tomb, for trembling and astonishment had seized them, and they said nothing"
The End