The general scholarly consensus is that the gospels of Matthew and Luke used the gospel of Mark as one of their sources. But Matthew and Luke also share content that is not in Mark, so it's believed that they both relied upon one more source, dubbed "Q" or the "Sayings Gospel". The idea, as I understand it, is that Matthew and Luke started with Q, a list of sayings attributed to Jesus, and then used Mark to provide supplementary information, like chronology and details of Jesus' life.
My question is, what is the scholarly consensus (if any) about which text is a more reliable source about Jesus' words, Mark or Q? Note that I'm not asking about the accuracy of Mark's account of Jesus' life and the like, simply the account of what he said.