They are similar but the meaning is a little different. "So you say!" This means you don't fully believe the other person. Depending on your tone it can be a bit sarcastic or playful/teasing. For example = A: "I'm really good at cooking." B: "So you say!" "That's what you think!" This is a stronger disagreement and is more direct. For example = A: "You are going to fail this test." B: "That's what you think." (You completely disagree) Hope this answered your question! :)