Miranda, Molecular Biology is certainly very exciting! You have an opportunity to learn about and participate in groundbreaking research that teaches us about who we are. Most biological sciences program provide rigorous curricula, it is difficult to compare whether one field is more rigorous than another simply because that depends on the student's sentiment. Be sure to look closely at courses required for various programs and see which plan may appeal more to your interests. You will likely find that the program that offers coursework in subjects you are really passionate about, isn't all too bad!