You are a long, long way from a buy back. You have only had them address the issue one time and you want a buy back for a new check engine light? Give them a chance to find the problem. When and if the issue repeats over several months and attempts to fix the car fail, then you can think about...
I think you should get the Prius. If the Mustang just doesn't speak to you, get something that does. For me, I don't care if it has a sound system or not, I'm not buying a car like this for the stereo. The music I care about comes out the rear of the car.