1) Don't plan your life away too soon. It's good to have an idea of where you want to go and what you want to be, but life changes a hell of a lot, so still be fluid in your ability to make decisions. Good on you for wanting to become a teacher, though!
2) If you want to become a teacher, you're going to need a lot of passion about your subjects. Every teacher I've ever spoken to owes enjoying teaching to the fact that they have always loved their subject throughout school.
3) Don't worry about this or that specific subject at this point. Have an open mind. Go to each lesson and think: "Could I see myself doing this for 30 years?", "do I understand this material enough to explain the concepts to a friend?"
Just enjoy the next few months and explore both of the options. Just go with the flow - the right decision will come to you eventually!