I actually had my Grandfather's 2001 Lexus ES300 Coach Edition as a first car.
I really did hate that car with a passion.
I like a driver's car, and that thing was the last thing to that. The steering was dead, and lifeless, they brakes were squishy, and too soft, and the ride was too isolated. I finally wrecked it after I bottomed out and destroyed the oil pan.
Overall, it was relatively reliable, and safe. But repairs were expensive. I guess if you like to be isolated while driving, this is your car.
The Mustang is the most expensive car for a teen driver to insure, tied with the Scion Tc, but it has the highest death rate among teen drivers. A convertible will increase your insurance even more, and it will be even higher if the Mustang has a V8.
Out of your two, I would take the Lexus because I don't want you to be another statistic among teen driving deaths.
But, if I were you, I would take a good look at the Hyundai Elantra. You can afford one new, they have unbeatable warranties, will keep a solid resale value, are safe, and highly recommended cars. Also, considering your age, Hyundai is notorious for being very lenient with giving out good financing rates for people with little or no credit.
I would take a solid look at it.
Here's a Picture:
Here's the Elantra's Website: