What is a better choice for a car? an 07 mustang for $9500 or an 05 ford 500 for $12000?

I am 16 years old. My parents bought a used 2005 ford five hundred about three months ago (the car now has 90,000 miles on it). We bought the car because my sister started to drive my fathers old car (an 05 highlander) which he loved. Now my sister is a senior, and is not taking a car to college, so in six months, we will have three cars for a three person family. My dad will take the highlander back, my mother will continue to drive our sienna, and i will take the 05 ford 500. Now my dad doesn't like the 500; which in fact we are trying to sell. We were told by one of our friends who owns a used car dealership that we could sell the 500 for a minimum of $12000 because of the shape its still in, and the cars have a great safety rating. I have a close friend who is selling an 07 mustang for $9500 that only has 12,500 miles on it. I know this sounds like a situation where a teenager is just trying to get a hot car, but i have looked at the specs and other than IIHS safety tests, the cars are close to the same statistic wise, and the mustang actually gets better mileage. Now the pros of buying the mustang are that we would save $2500 and get a car that gets better gas mileage, and has much less miles on it. The cons would be that the insurance would go up, and we would get a car with somewhat of a lower safety rating. I still think that (besides how the car is beautiful) the statistical pro's outway the cons. Please give me your opinion, I would greatly appreciate it.