What should I do? What would you do if you were me?
Being a man of principle, I would honor your deal with the plane for $21,000. How many potential sales has this buyer lost holding this aircraft for you? And he has done so without asking a down payment. You owe it to this buyer to see this transaction through.
But otherwise, I still think cheaper plane is still the better deal for you. $25,000 for any standard 150 is too much money in today's declining market. Especially if you do not plan on keeping it for any length of time. You will have a lot better chance of getting your money back paying $21,000 over $25,000 for a Cessna 150.