At this point in my life, I like teaching MCAT much better than teaching SAT. But I'm in a different position than you are. It's important for you to be able to maintain control of the class and be the "authority," so to speak. This is difficult when you're a junior teaching your classmates from school. I really advise you to wait until you at least are a senior if not a college graduate before you move into MCAT courses, because it will really help you in terms of how the students perceive you.
Teaching in the private sector is very different than TAing at a university. The students are your customers, and you as an instructor are their employee. If they aren't happy, you aren't going to be around for long. Since you are being rated by students, you always need to consider how you come across to them. I don't know what TPR does, but at Kaplan they have students rate you at the midpoint and again at the end of the class, and your ratings affect the future classes you get assigned as well as how much you get paid. I had a very unfair advantage for several reasons: I scored much higher on the MCAT than any of my fellow instructors did (which some students unfairly assume makes me a "better" instructor), I was older and more experienced at teaching than many of the other instructors, and I was a grad student and therefore perceived to be an "expert." These kinds of things definitely matter to the students. It's not fair, because you can't control how old you are or change your MCAT score, and it's not worth going to grad school just to teach at TPR. But it's how things are.
Teaching MCAT is very challenging, but that's why it's my favorite class to teach. First of all, the students are sharp, very sharp. They will keep you on your toes. No two classes are ever alike. Many of the students are paying for the course themselves, especially non-trads, and they will expect a lot from you for their money. You need to know the test prep techniques cold, and you also need to know the science cold. It takes a lot of preparation to teach MCAT classes. It's also challenging to keep the class entertained and get them to participate. Many of my students were accustomed to enormous college classes where they'd sit in the back and not say anything. How you conduct classes will depend on your teaching style, but mine are a bit of a free-for-all. I like getting the students worked up and having people shout out answers instead of just sitting there listening to me talk. I think that makes the class more enjoyable for them as well as for me.
Teaching SAT is fun too, but in a different way. First of all, the students are high schoolers, so they have a shorter attention span, and you have to make sure to get them to participate. You have to be careful about how you criticize them, since they are more sensitive than adults. You also have to be stricter about them doing their assignments, since most of them are underage and you are actually responsible to their parents. Dealing with dissatisfied parents is the most difficult part of teaching SAT. Teaching high schoolers as a college student is fun. They really look up to college students, just like college students look up to medical students. But at the same time, you're not so much older than they are that they feel you're like their parent or something. You're still "cool" to a high school student. To be successful at teaching SAT, you must know the strategies cold, and keep the class motivated and on task.
Talk to your TPR center about teaching SAT for a while, Eddie. I promise that it's a much better place to start than MCAT for a college student like you. After a year or two, then you can start on MCAT too. You'll be more experienced at teaching, and the students won't be your peers any more. Things will go more smoothly for you, and you'll be a higher-rated instructor if you do this.