A Flat tax never stays flat. The US has had several flat taxes in its history. Always the same result; grows into a monstrosity.
Go with the FairTax. And don’t criticize it until you’ve actually studied and understood it:
http://www.fairtax.org/site/PageServer?pagename=FAQs
I have studied the fair tax and consider it far worse than a flat tax and on par with our current tax mess. No thank you.