That Psalm does not say that David is the Begotten Son of God. Stop spreading lies! If you read it in it's context, it is clearly talking about Jesus. Did you even care to bother reading the last verse in that Psalm? Did you read Psalm 2:12? Do homage to the Son, that He not become angry, and you perish {in} the way, For His wrath may soon be kindled. How blessed are all who take refuge in Him!
What other "son" of God would have such power? If this was David talking about himself, then why would he say in verse 12, "Blessed are they who take refuge(put their trust) in him(the Son). It is clear and so obvious that it is talking about Jesus. You know when something is a figure of speech or when it is intended to be literal. In this case, there is no figure of speech.
So as you can see, if this Psalm shows(along with other parts of scripture) that Jesus is the Only Begotten Son of God, then there is a difference between being an Only Begotten Son of God, and a son of God. Because we are sons or daughters of God, do people put their trust in us? It is in the Only Begotten Son of God that people put their trust.
Also, let us look at Isaiah 9:6 to see the difference between Jesus as the Son of God and others as the sons of God: "For a child will be born to us, a son will be given to us; And the government will rest on His shoulders; And His name will be called Wonderful Counselor, Mighty God, Eternal Father, Prince of Peace."