See they are two different streams so there is nothing like a best option. I believe you should do what you most likely would enjoy studying because this is three years of graduation and to get good marks, you should like what you are studying. It is easy to say B.A or B.Sc but that is definitely not how it works. One is arts and one is science. You should take up what you like - B.Sc if you like science and B.A if you like arts. Study for graduation and prepare for UPSC alongside. Civil services is conducted in so many subjects - check the complete details here as you should know and prepare for this too. here http://www.upsc.gov.in/sites/default/files/Engl_CSP_2017.pdf