The Kingdom of God is a far more important subject, because that's the gospel that Christ actually declared. Jesus is not the most important subject there is. The Father is. Even after Jesus was resurrected, He continued to preach His Father as being the most important subject.
It seems like nowadays the Father is only important enough to mention when someone wants to defend the trinity doctrine. Jesus is the Head of the Church, but the Father is the Head of the Family. As Jhn 1 says, Christ came to declare the Father. Jesus had an extremely monumental role to play, but a lot of "Christians" seem to forget that Jesus said that He was a Messenger, and a messenger is not more important than, or even as important as, the party that sent them.