OK...I have VERY mixed emotions about my answer for this thread.
Immediate knee jerk answer would be-PERFORMANCE. Chrome don't get you home, and shine won't be first at the line. PERIOD!
BUT(and don't take this personally, it is just a fact of life )
I would say that probably 110% of high school aged people ( there, feel better? I didn't say "kids" ) are pretty certain that they are bulletproof, know everything, and are the best at everything. Question one, and he/she will tell you that maybe yes, they know some people that aren't, but they are different than those people and they are not like that...
The Mustang, while not a high dollar exotic, or a Vette, or something along those lines, is a performance automobile. I don't care how good of a driver you are, or how careful of a driver you are, or how well you did on your drivers education exam, or how young you were when daddy let you sit in his lap and steer while he worked the pedels. Bottom line, you have not been driving long enough to be as good as you think you are. Given the opportunity to display a little testosterone you are probably going to do so. Then you are going to realize what a rush that was, and do it a little more often. Even if it is as something as innocent as punching the gas going around a corner. You are going to do it. Increase the horsepower, you are going to be increasing the odds of the inevidable, you crashing it, possibly hurting yourself, worse yet, hurting someone else. So for your perticular situation, I would definitely say go for the appearance mods for right now.
Like I said at the beginning, don't take any of this personally. We have all been there. Hell, I am still there!