Dear Confused Guy:
Let us first understand the difference between an MBA and an MS degree. An MBA offers formal management training in a systematic and structured fashion. Thus, it is aimed at training future business executives. As an MBA graduate, you would secure a job in the management track that would expose your to various aspects of the business including: day-to-day operations of the business, managing people and other resources, delivering value to both the customers and shareholders, business planning and decision-making, and so on. Your performance, and thus success, would be determined by your ability to deliver demonstrable results that contribute to the growth and success of business.
An MS degree is a further specialization in a particular field, offering more in-depth knowledge in the field. Thus, it is aimed at producing specialists or experts in that field. With an MS, you would start your career in your particular field of expertise. Your success would be determined by your knowledge of the field and its application, and your continued learning and innovation in that filed. As you climb up the ladder, you would become a recognized expert in the filed, as opposed to a business manager as is the case with an MBA.
The purpose of the two degrees is quite different, and you must recognize and understand the difference.
It is important for one to know one's interests and ambitions. If one does not know where one is going and why one is going there, then it is very difficult to know what "there" is. You are not alone in your struggle to identify, let alone finalize, your ambitions and interests. It is a lifelong quest; the searching and groping continues throughout one's life. It is not uncommon anymore for one to change careers along the way, as new interests develop or current interests take on sharper definition. You should not be paralyzed by the prospect and/or desire to settle your ambitions and interests once for all.
There is no magic formula. We don't know you and you don't know us, so it is not feasible for us to even try to determine what is best for you. There are tests one can take to theoretically determine one's ideal career. We don't put much stock in such tests. Given your life experiences thus far - education or work - you must have enjoyed indulging in some things more than others. That is a start; it begins to offer some insight into your interests. In terms of your ambitions, you require some work experience, if you don't already have some, to learn what you would like to become sometime in the future.
Most good MBA programs require candidates to have 2-5 years of work experience. With work experience, you would have a better idea of whether an MBA is for you.
A final thought: why limit yourself to a this-or-that choice? You could do both. You are young and you have options. Good luck!