The flat tax has a fatal flaw--politicians. The income tax started out flat, but ballooned to over 50 brackets at one point. After Reagan flattened it into two (or three) brackets, it has bounced back to 6 or 7, depending on how you count them.
Politicians WILL NOT let a flat income tax stay flat. They can't.
The fair tax has an unbeatable advantage over any income tax. That's in addition to openness, fairness, and simplicity. Under a consumption tax, the government has no business knowing how much money I make, how much I spend and on what, who much I save, and in what kind of accounts, or how much I inherit or receive or give as a gift. None of that.
The Fair Tax gets the government out of our personal finances entirely.