Happy to know about your interest for IAS. IAS is Indian Administrative Services and it is a government job. The eligibility criteria for this does not mentioned about any particular specialization for degree courses and candidates are free to pursue any as per their choice. The same is applicable for you too. You can choose B.Sc. or can prepare for IIT examination as per your choice.
My advice for you is to go for some courses related to our society, politics and our country. This will make you understand more about what you will be doing in future. This knowledge will also help you in UPSC examinations in future. If your goal is clear and you are well determined for Administrative services you should not waste your time in preparing for other exams and better go further in the same direction of IAS.
If you will pursue a degree in Sociology, Political Science or any other related stream, the preparations for UPSC examinations may appear easier for you.
All the best!
Living & Learning- simultaneous processes!