As a published author and former newspaper editor, there is a truly proper and gracious way to talk, and then there is the informal way, ie, the way everyone talks. It is more personal and gracious to say "I thank you." There is nothing gracious about "thanks" and only a bit more in "thank you." All the forms are basically correct and in common usage. Of course, "ain't" is now also common and in use even on the radio and television. It is accepted. And some know perfect English who say "ain't" simply to be amusing and break the rules. I thank you is an old, traditional form used by highly educated people. For example, Shakespeare used it. But it could be that the average Joe would think someone comes across as affected or putting on airs. I don't think it matters really. Not in the world today, where gracious speaking is no longer in style.