I'm giving you a torch that will shed light on which subject you should choose. You can have faith in this torch of mine. Although I have called this my torch, the battery in it belongs to UPSC. In discussing the syllabus of the mains exam, UPSC has already clearly mentioned that "The main examination is intended to access the overall intellectual traits and depth of understanding of candidates rather than merely the range of their information and memory." It is then clear that the subject is not important; your knowledge ofthe subject is what matters. When you start being analytical about a subject, you gain a deep knowledge of it and start developing your thoughts accordingly. This renders even a simple subject important. On the other hand, even the most important subject can be turned simple.
So use this torch when you are deciding upon a subject for yourself. Do not be in haste when it comes to choosing your subject. Think about it calmly. You can take advice from other people, listen to what they have to say, but do not get so influenced that their suggestion becomes your decision. Think about it logically, analyse your facts and then take a decision.
Once you have made your decision, stick to it, do not change it easily. Alter your decision only if you cannot fall in love with the subject, or cannot get a complete hold of it despite trying everything. Or you should change your decision if you feel that you love your subject, you have a good hold of it, but the exam results are too bad for you. I'm not talking of exam results being bad, I'm talking of the results being really bad. If your results are too bad, do not blame UPSC or the paper; do not blame the books you have studied or the examiner who corrected your answer sheet. Instead of finding faults in all the other things, find out where you were lacking. By blaming things, you will get a false sense of self-satisfaction, but not success. It's up to you now, whether you remain happy with a false self-satisfaction or should you opt for real success. If you want success, work on your own shortcomings because you do not have control over UPSC or your examiners.
I'm mentioning this because I have seen students change their subject. They do not change themselves, they keep changing their subjects and consequently their results do not change. As I mentioned above, students get selected from every subject and if we are not getting selected the problem lies with us.
However, there is one more truth. If you are not gettingthe hang of a subject initially, you should not take too long to change it. Even if you have wasted a year on that subject, it is better not to waste five more years over it. This will not just save your years but also your future.